Here's why:

* Fissionability: Uranium-235 is fissile, meaning it can undergo nuclear fission when bombarded with neutrons. This process releases a tremendous amount of energy, which is harnessed in nuclear power plants.

* Abundance: While Uranium-235 is only a small fraction (about 0.7%) of naturally occurring uranium, it's still the most readily available fissile material.

* Neutron Production: Fission of Uranium-235 releases neutrons, which can then cause further fission in other Uranium-235 atoms, leading to a chain reaction.

Other radioisotopes can also be used in nuclear fuel:

* Plutonium-239 (²³⁹Pu): This is a byproduct of uranium fission and can also be used as fuel.

* Thorium-232 (²³²Th): This is a fertile material that can be converted to fissile Uranium-233 through neutron capture.

However, Uranium-235 remains the primary fuel source for most nuclear reactors.