Because you have a cycle of ~14 days of darkness and light on the moon so solar will need a backup.

Batteries are an option but 14 days worth of storage for a lunar base is probably asking a bit too much.

Also nuclear provides another benefit on the moon - heat which you’ll definitely need during those 14 days of darkness.

In addition to what others have said, in the places where the water is and the energy is needed, there is no sun. So you need to first go to a place where there is sun, then beam energy into a crator.