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.

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Happy Trails,
Clint.