Same reason radioactivity was the explanation for everything post ww2 I guess? Everyone knew of it, knew it could mutate DNA and stuff and it was an easy explanation that was more real, seemed to be based on science than like gods and people from other planets and stuff. All movie monsters in the 50s were giant and mutated because of radioactivity of some kind, it was an easy way to explain away fantastic things and super power I guess. Mostly a product of the times it seems to me. Was there any radioactive superheros created post 1960s and the height of the cold war? Probably not.