Sometimes not purging all your best people can work out well for you

"He did write a letter to Roosevelt about atomic energy. I think this was in part his agony at the evil of the Nazis, in part not wanting to harm any one in any way; but I ought to report that that letter had very little effect, and that Einstein himself is really not answerable for all that came later. I believe he so understood it himself." -Robert Oppenheimer speaking about his relationship with Albert Einstein. Oppenheimer’s lecture was delivered at UNESCO House in Paris on December 13, 1965. The text was reprinted in The New York Review of Books the following March.

Einstein was never actively involved in developing nuclear weapons, as he was deemed a security risk due to his German origins. He laid the scientific groundwork for nuclear fission, which served as a basis for the atomic bombs.

Some have additional rumors as to why he was never invited to work on the Manhattan Project, from some claiming he was a fraud, a plagiarist, the people already on the project didn't like him, or that his personal beliefs on the end goal of the project would interfere or impede it's progress.