US tank doctrine,tank forces should be divided into 2 groups.

Heavy tanks attached to infantry divisions would be used to make breakthroughs and penetrate fortified lines.

Armoured divisions supposed to penetrate behind enemy lines shooting up enemy artillery and such...
a job the Sherman was OK for.

But they had to make the breakthroughs and penetrate fortified lines as there were no 'heavy tanks'.
So here the German's,under constant bombing,come up with Panthers,Tigers,Jadgpanthers,StuGs, etc.

Russia comes up with T-34s, ISUs, etc.

The U.S.,with all her industrial might...comes up with the Sherman.