Why is uranium 235 the most commonly used isotope for nuclear fission in current uses in nuclear power generation?

This is because U-235 decays naturally by a process known as alpha radiation. This means that it releases an alpha particle (two neutrons and two protons connected together).
Another reason that U-235 is ideal for producing nuclear power is that unlike most materials, U-235 can undergo induced fission. When a free neutron collides with a U-235 nucleus, the nucleus will usually capture the neutron and split extremely quickly. The splitting of a single U-235 atom can release roughly 200 MeV (million electron volts).