Comment by jcranmer

Every element up to and including bismuth, excluding technetium and promethium, have stable primordial isotopes (i.e., have been on Earth since it was made), and in addition, thorium, uranium, and maybe plutonium (Pu-244 is on the very edge, so it's not clear if any primordial nuclei of it remain) also have primordial isotopes. Every element with bismuth or higher atomic number has no stable isotopes, and the elements from astatine through neptunium naturally occur largely via decay sequences of uranium or natural nuclear reactions in uranium ore.

Nuclear fission reactions tend to result in the daughter nuclei being considerably smaller than the mother nucleus--like a 70/30 or 60/40 split, which means that the fission products of uranium are firmly in the range of elements that have stable isotopes. (Although due to larger elements being richer in neutrons, most fission products have too many neutrons, hence undergo radioactive decay themselves).