I don't know the exact reasons why Vikram didn't get a fission reactor. But I can assume from similar missions:

1. Solar is pretty good as far as Mars and it gets worse as it travel further from the Sun. This is why most probes that travel past Mars need a nuclear reactor (Voyager, Pioneer, Cassini, etc). Going closer to the sun they get even better

2. Sending radioactive materials on rockets presents a risk and it is avoided if possible, lunar probes are usually cheaper and can still benefit from solar, so no need for nuclear. Imagine throwing plutonium in the atmosphere in the case of an accident

3. Nuclear reactors in probes are small and rely on decay radiation, they _usually_ have pretty small powet output, solar has a lot

4. And last but not least, price, solar is much cheaper than nuclear