>>514918048
For the same reason that Germany did not swap over to entirely jet powered aircraft. The re-tooling of machines for manufacturing parts and the training of mechanics and fabricators would have been a logistical nightmare. The U.S. is fricking huge and this sort of a deal needs to be done very deliberately and slowly.

The U.S. does use metric for military and scientific pursuits and whenever they really need something done precisely. It's simply industry and that populace at large who use Imperial.