Why only Uranium is used to make Atomic Fuel?

There are 3 natural isotopes of Uranium. Uranium 238, Uranium 235 and Uranium 234. Among these, the nucleus of Uranium 235 is fissioned quickly by the hit of neutrons. Thats why chain reaction can be possible with this isotope easily. Moreover, there is no other isotope of any primary element found in nature that can be fissioned easily. There are some fissionable isotopes like- Uranium 233, Plutonium 239 etc but cannot be found in nature. They can be found in atomic reactor artificially. They are called secondary isotopes. On the other hand, Uranium 235 is primary isotope, which is irreplaceable to produce atomic energy.