This isn't entirely true. The US was fully aware of torsion bar suspension systems like those used in the Tiger, Panther and, Pz III. The US also knew about the Christie system using large diameter vertically coil sprung suspensions. The US Army's ordinance department however settled on the use of the volute spring suspension system as their standard demanding its use on designs up through 1942.
Thus, the Stuart, Grant and, Sherman all used the volute suspension system. For vehicles capable of speeds up to 20 mph cross country this system was adequite.
On the other hand, claiming the Germans that "the Americans [were] in the in the war with a tank far inferior to the high standards set by other tanks..." is an absurd claim.
The Sherman was a well thought out for 1940. It had a large enough turret ring to allow easy up gunning and new turrets that could take larger guns. The track system was far superior to anything the Germans used on their tanks in terms of life (about triple that of the German or Russian designs) and at road speeds reduced gasoline consumption giving tanks better mileage when travelling out of combat areas.
The US also continually improved their metallurgy making their armor perform better for a given thickness than that of Germany or Russia too. US tanks also had excellent radios, the best turret power traverse systems, the first operational gun stabilizer systems (and, yes, they did work when crews bothered to learn to use them). The only real drawback to the early US designs was too narrow a track (something the Germans were also guilty of on their early tanks) and too high a silouette due to the original choice of radial aircraft engines for power plants.
With the second generation vehicles like the M-24, and M-26 these problems were rectified and these vehicles were as good as anything the Germans produced. The M-25A1, of which 300 were produced prior to June 1944 but never saw combat, was essentially a US 'Panther'tank. It had equal or better armor, a 90mm gun, equal speed and cross-country performance and, was better equipped in details than the Panther. The ordinance department decided to by-pass its production in favor of the M-26 based on promises of the manufacturers to get the later in production within a few months (which, unfortunately, did not happen).
The same went with a plan to equip M4A3 Shermans with the M-26 turret as an interm vehicle just before D-Day (both tanks have the same diameter turret ring....thanks to good design and engineering).
As for the T-34 it had good armor a decent gun and good automotive performance on paper. The reality was it was severly handicapped by its 2 man turret and lack of vision devices which left it virtually blind except to events happening right in front of it. It's tracks and drive train were riddled with bugs and took several years to make reliable in service. The ammunition layout left much to be desired. All but 3 rounds were stored under the floor of the turret (which also lacked a basket) making loading slow and difficult. Lack of a radio and intercom system also made the crew less efficent.
Basically, there is alot more to a tank than just its gun and armor. It is a complex weapon system that has many more parts that make it efficent than just its gun and armor. On that basis, the US didn't do any worse than any of the other major combatants and, in many ways did alot better.

There really wasn't a single reason.
Even though the M26 Pershing had been in development since 1942, as an upgrade to the M4 Sherman, it was an on again off again project. Which is why it wasn't’ until late Feb. of 1945 that the first of them saw combat. Now while people sometimes think they used the same Ford aluminum, double overhead cam engine as the Sherman, it was really a redesigned engine to shrink its overall height while gaining about 50 horsepower. The Pershing had the GAF, while the Sherman had the GAA.

This added to the stress on the heads, and consequently they had a tendency to fail. Also, it was coupled to an automatic three speed transmission, instead of a manual "gear cruncher". So it was a "new" engine, a "new" Torquematic transmission, and riding on a torsion bar suspension. A great many "firsts" for American armor.

Due to its wider tracks, it actually had about the same ground pressure as the Sherman, 12-13 pounds per square inch. But its weight cut its mileage to about 0.5 mpg. That is only slightly less than the Sherman in the final "jumbo" version, but still gas mileage wasn’t a great concern to the Allies and with its larger fuel capacity the range was about the same on identical surfaces.

New engine and power-train designs rarely, if ever, succeed in their first incarnation.

__________________
Happy Trails,
Clint.

In a word, no. You are overlooking the difficulty in deploying an expeditionary force that needed to be trained and equipped in America, shipped over the Atlantic Ocean, docked in England, then transported by rail to South England where they would reassemble and put into ships again to cross the Channel. Imagine all this military and logistic chaos taking place at the same time the very tanks, guns and trucks to be used in the coming battle were under going designing or being qeued for production. To introduce the Pershing tank into the intricate plans would be like performing a heart transplant while the patient is undergoing neuro surgery. In fact, in May 1944, there were ten T26 prototypes in the US Army, and they were all state side, less than one month before the Allied armies start an amphibious strategic offensive. Mind you, amphibious strategic offensive was a type of operation that military strategists pronounced impossible at the 20s-30s because of the failure of Gallipoli. This was to be practiced on the Germans, who had the best army in the world, at the part of Europe that was the best defended real estate in the world. To get any servicable tanks on the beachhead, rapidly and in overwhelming numbers, were of far greater import than what tank they actually would be.

Let's just stick to mobility for the moment. Your stats above are "book" values and of little relevance on their own or outside the world of engineering test tracks.
When it comes to off road vehicular mobility there are two broad categories that really matter: Agility and flotation.
Agility is automotive performance. Acceleration, torque, tractive power, that sort of thing. Flotation is how the vehicle's wheels, tracks, or whatever interact with the soil / ground under them. These two factors in combination are what really matters in determining an AFV's mobility at a tactical, and sometimes operational, level.

For example, the Sherman uses a rubber bushed double pin track that is tensioned and has return rollers. While this track is not as efficent at low speeds, say under 15 mph it is actually more efficent energy-wise at speeds above that. The result is that a Sherman driving on a paved road (like in a long road march) is more economical and has less rolling resistance than a tank with a steel track lacking return rollers.
Steel tracks also have the bad disadvantage of being very hard on most paved road surfaces. These tracks will generally tear the road up in short order. This too is a problem particularly if there are follow on units needing to use the road.

Then there is the issue of straight weight. Heavier vehicles are restricted on what surfaces they can move and this is true to an extent regardless of ground pressure exerted by the vehicle. Here, a very heavy tank like the Tiger is at a disadvantage. It will damage all but the most heavily built roads (and in the WW II era there were few roads designed for a load use of 50 tons or more). It is restricted on what bridges it can cross (one reason the Germans increased fording capacity).

Acceleration is how lively the vehicle is. How quick it can go from a standing start to whatever speed. This is important for a combat vehicle actually. A smooth and quick acceleration allows a crew to move out of danger more rapidly. A higher horsepower to weight ratio also says something about torque and maneuverability or "liveliness" of the vehicle.

Anyway, what you find is that the Germans and US as two examples had very different ideas about what tanks were for and what criteria they were looking for in them. Clearly the Germans were far more concerned about the defensive capacity of their tanks. They sought long range gunfire and antitank performance. Their armor layouts more and more reflected a view that engagements would be head on. There was less concern about operational mobility that would be required in a deep offensive operation or breakthrough.
The US on the other hand concerned themselves more, at least initially, with operational mobility. Their tanks were optimized for use in breakthroughs and not tank on tank combat. Good road and high speed automotive performance gave them what they were looking for. Excellent maintainability and reliability added to this too.

Then the US coupled this with an unmatched civil engineering capacity. That is they could build roads where none existed. They could bridge rivers of any size in hours (the 291st Engineer battalion put three bridges across the Rhine in less than 24 hours as one example). The inclusion of masses of heavy construction machinery in engineer battalions and dozer tanks in tank battalions made a huge difference.
For the Germans, if a bridge that could take a Tiger didn't exist for a river crossing it might be a week or more before the crossing could occur if the tank could not ford the river.

Soils are another factor in the mobility equation. The Soviets set their tank design ground pressure for all but the heaviest vehicles at about 9 psi, at least initially. This value was chosen as it allows mobility on saturated chernozym soil common in Russia. Above this value a tank would break the crust of the soil and become bogged in the muck below.
Thus, the repeated German surprise at the Soviets operating tanks in places they assumed tanks could never go.

You see, I see you as looking at this like sports statistics. How many touchdowns, home runs, plays made etc., by individual players. But, tanks don't operate in a vacuum. They are part of a larger system in modern warfare. It is the system that needs examination. On the part of the Germans they failed to appreciate the need for a good maintenance and civil engineering system to go with their mechanized units.
Instead, they optimized their equipment for great tactical performance. The US and Soviets optimized theirs for great operational performance as part of a much larger system. The result was the Germans could win locally more often than not but ended up losing the larger battle virtually every time.
It was a design flaw in itself that their doctrine had them producing the wrong kind of tanks for the war they were fighting.

Tigers in trouble...

" Rolf Ehrhardt experienced a tank attack on the east edge of the road to Roanne:

A Number of Shermans approached on the road on the other side of a ravine. The American unit was recognized early enough and we let it approach to a favorable position. After the first miss of one of our Tigers, the Shermans unexpectedly formed a front and opened quick fire. This forced me to flee to the basement of the house where I had an observation post on the second floor. The rounds coming from our Tigers were noticeably different than the incoming. Every shot fired by an 8.8 cm was a hit in our thoughts. Suddenly the Tiger commander, Untersturmführer Hantusch, collapsed in the basement, both hands pressed to his head. His Tiger had received numerous hits which shook up its weapon system, and the electrical power was knocked out. After another round to the turret he was wounded on the head and had to abandon the smoking Panzer which could catch fire at any minute.

Minutes later the second Tiger Kommandant, Obersturmführer Dollinger, came back and was bleeding heavily from his head and was silent. After his wound had been dressed he reported that the smoke from the rounds made it impossible to see through the gun´s optics. It was impossible for him to see his targets. The thick response of enemy hits ruined the possibility of hitting another Sherman.

The numerical superiority of the approximately fifteen Shermans turned our superiority in weaponry into nothing. The Tiger of Obersturmführer Dollinger then received another hit which "amputated" the first third of its cannon. This caused us to realize that we had been literally crushed, which had never before been so obvious."

From " Chronicle of the 7. panzer-Kompanie I.SS-Panzer Division Leibstandarte" by Ralf Tiemann

__________________