Short answer: the idea of atomic bombs and atomic energy as a science fiction, future-science trope was very common prior to the use of the atomic bomb. The idea that they actually could exist much less did exist was not — that was the big surprise. The analogy I use is like a "warp drive" today: nearly everybody has some idea of what I might be talking about if I say it (something about faster-than-light travel, and if you are the kind of person who cares about science then you probably know that there are only a few ways that might be accomplished with our current understanding of physics), but you would be VERY surprised if not only it was announced that it was determined to be totally feasible but in fact had been put into use yesterday.

Longer answer: see this previous answer, which goes into detail about the popularization of the discoveries of radioactivity, fission, and so on.