Why is fission not an important energy source in the Sun?

Nuclear Fission -

The process of the splitting of atom, by radioactive decay process or by any collisional impact.

Sun contains many radioactive isotopes like, thorium, uranium, etc.

The nuclear fission process is an intrinsic property of the radioactive nuclides.

Nuclear Fusion -

The process, where two or more than two atomic nuclei combines to form one or more different atomic nuclei and subatomic particles.

The energy released in both the process is almost same,

But on the surface of Sun, hydrogen is abundantly present, hence, hydrogen fusion is possible, so the dominant source of energy on the sun is Fusion.