Here's why:

* Fusion: This is the process where lighter atomic nuclei combine to form heavier nuclei, releasing immense energy in the process. In stars, primarily hydrogen atoms fuse to form helium, releasing energy as light and heat.

* Fission: This is the process where a heavy atomic nucleus splits into two or more lighter nuclei, releasing energy. While fission is the primary energy source for nuclear power plants, it doesn't occur in stars to the same extent.

Stars are massive enough to create the intense pressure and temperature needed for fusion to occur in their cores. Fission, on the other hand, typically requires a neutron to initiate the splitting process, which is less common in the environment of a star.

So, while fission does play a role in certain stellar processes, it's not the dominant energy source like fusion is.