Probably because Einstein was Jewish. It was a good thing for us that he was Jewish, too -- he might have made a significant contribution to Uncle Adolf's A-bomb program otherwise.
Einstein was not that kind of physicist.
Once you write down E=mc2 you've pretty much got all of the nuclear weapon you're going to get from him. The theory of a nuclear weapon is trivial compared to the materials handling, metalurgy, physical chemistry, ordnance, electrical, mechanical and other engineering challenges you have to solve.
As a pure theorist, Einstein would not have been a lot of help.