Isn't one of the key aspects of laser and plasma weaponry in lore that it is easy to maintain and operate? No bullet drop, no recoil, no need for oil, no moving parts to get sand and dust clogging.
Also, I kind of assumed that the Fiends' energy weapons came from Caesar. The Legion never use energy weapons, somebody in-game says that he suspects the legion are stirring up the fiends, I put two and two together and made 5, and thought that any energy weapons the legion scavenged were distributed to groups like the fiends who were disrupting NCR operations. I was actually quite surprised when the whole Legion-fiends link was never played any further than that throwaway comment.