Why are uranium and plutonium the only radioactive materials regularly used to create nuclear chain reactions?

It seems that the only materials used regularly for nuclear chain reactions are uranium and plutonium. Why, exactly? In principle, any radioactive material could be used to create a chain reaction. And, this would open up a wide variety of potential options for nuclear reactors, and, also, the possibility of cheaper and safer nuclear explosives that could then be used for civilian commercial applications, and, not just as military "bombs". There are a wide variety of naturally occurring radioactive elements, and, potentially, a limitless number of synthetically created radioactive isotopes of elements. Why not try some of these, for the creation of nuclear chain reactions? https://en.wikipedia.org/wiki/Fissile_material