AT guns are simply too costly compared to how easily they die, and because they need vet for AP they are unusable against heavy tanks. The reliance on vet is flat out retarded for something that dies if you look at the wrong way. The COH1 AT gun had AP right out the factory.
Well, so does this 57mm AT gun in CoH2.
It's cheaper as well, just 30 munitions. The Vet 1 ability isn't the AP shells, it's the "Take Aim!" which gives farther sight range and farther firing range (which is damn awesome), at the expense of sight being restricted the cone of fire.
I wouldn't necessarily call the M1 AT Gun the best AT gun in the game - I still personally find the PaK 40 (when Target Weakspot behaves) to be the best AT gun in the game. But back to the USF AT. :3