I find is amazing that we had so much fissionable material in 1945. How did we get so far ahead of the Germans?
We kidnapped them from SA.
Because a very large percentage, probably a majority, of the top physicists of Germany and Europe had fled. They were Commies and/or Jews and life wasn't pleasant for them.
Germany was also stuck on a small continent with little radioactive ore. We controlled the seas.
USA also had a great deal more money and productive capacity. We could afford to divert massive resources to something that might or might not pay out.