The Enclave was actually pretty destroyed in the end of FO2. In the west coast. So it makes no sense for them to be hanging around there. Remnants at best.
I disagree. It made no sense that an organization, a self proclaimed government that powerful and methodical could just have one operation and keep everything on one dinky lil oil rig in the middle of the water. The Enclave were way too paranoid to leave everything to chance like that.
It makes perfect sense to me that the Enclave would have contingency plans and different cells popping up over various parts of the country, like their attempts to claim the capital.
That being said, I'm not upset they're not really in New Vegas. They were a huge part of FO2 and FO3, it's time for them to take a break and let newer factions like Caesar's Legion step in for awhile.