gunnergoz already mentioned the crew quality.
Don't forget the M4A1 went into production by March 1942, but the T34/M40 2 years earlier. T34 had a way superior cross country performance and a diesel engine. When the Germans came along the first M4A1, they nicknamed it "Thommy-Kocher" (Brit Boiler free translation) because of its tendency to catch fire easily.
I think the main downpoint of the T34 in the game is really the quality of its crews.