The Jumbo Sherman- Why weren't all US tanks outfitted like this?

If I recall correctly only 60 or so of the original Jumbo's were destroyed. Together with the expedient Jumbo's that were created in the field this amounted to a fairly sizable "heavy tank" force.

 

In July 1944 they wanted a pilot to be built on the M4A3 mounting the T26 + 90mm.

Later after the success of the Jumbo they felt there was now a definite requirement for an assault tank. They were unsure what form it would take, Additional M4A3E2's, The T26E5, or a heavied-up turret and mount on an M4A3 hull with E8 horizontal volute suspension.

In order to see how the E8 suspension would stand up to this they ran a 2,000 mile test on an M4A3 hull with E8 suspension, carrying a T26 type turret mounting the 90mm gun. The all in weight of this tank was 102,000 lbs. Tests happened towards the end of 1944 start of 1945.

 

Well this is pleasing.
Rather thorough testing of the transmission armour on an E2:



That is quite a hefty lump of metal...

 

Seems like 90mm M82 was the way to go if you could catch the tank unangled. A long 88 would have sliced through at any obliquity and from pretty decent range.
Of course the long 88 could sliced through any tank at just about any angle at most ranges.