Riain,
You keep stating that the US could have done it. Agreed, the US could have fielded heavy tanks during WWII. But you've not answered why.
The US and the Western Allies were winning the war without fielding those heavy tanks. As has been pointed out, developing, fielding and then supporting those heavy tanks would not have been a trivial exercise and would have consumed resources otherwise spent on fielding even more mediums and other weapons.
As the Western Allies were already winning the war without the use of heavy tanks - why bother with them?
Sure, they spent some efforts on developing prototypes on a "just in case" basis but beyond that the Western Allies never saw the need to get them to the field. Commanders in the field were screaming for more mediums - not heavies. And those commanders were also screaming for more high explosive rounds for those mediums, not anti-tank rounds, as HE was better at the infantry support missions those tanks were more frequently employed in.
Thus there's no compelling answer to "why" the Allies should have bothered fielding heavy tanks.