Because Shermans are cooler (and were WAY more numerous).

That and the vast majority of American tank freaks, deep down, want to fight the Battle of the Bulge more than anything (and probably not an insignificant number of Eurpoean tank freaks). From D-Day to the fall of Germany ... more Shermans made tracks on European soil than any other western front MBT.
There are many tanks that some could claim were better. There are a few tanks some could claim "had a significant impact." But on the western front ... there is not a MBT (granted, various chassis and setups) that anyone can claim was used more by the allies.
The M4A1 (first accepted in February 1942) # made: 6281
The M4A2 (first accepted in April 1942) # made: 8053
The M4A3 (first accepted in June 1942) # made: 1690
The M4A3E2 "Jumbo" (June 1944) # made: 254
I'd recommend the A1 or A2 to be the first allied MBT followed by the T-34 for the Eastern front. Then later add the Jumbo Sherman as a perkie tank for the allies.