Well, the Mojave desert used to be quite secluded and isolated until 6-7 years before the events of New Vegas (when Mister House took control of the Strip, basically, if we are to believe the ghoul mercenary from Fort Mormont). No international power took interest in the land for a long time, so it kind of makes sense that the currency wasn't backed by a federal reserve until then. Wasn't populated or connected enough to form their own regional power. Under Mister House's dictatorship, however, it's possible that the Mojave will, in time, use its own currency. Until then, they were still the direct neighboors of the NCR, so the local currency simply exported there thanks to the water merchants, and simply never changed until now. Secondly, unlike Washington and Boston (which are built on a coast, hell, built on freaking swamps with clean ground waters), the Mojave is a desert. The original idea of the caps (one cap = one bottle of water) makes sense there, until comes a power who will shift the economical status and provide its own currency. Which is exactly what happens with the Legion's daenarii and the NCR printed dollars. So, they still use caps, sure, but at least there's a reason why. Finally, the game tells us many times that this currency is about to die out. The crimson caravan mentions how unstable it is, because of how easy it is to counterfeit caps, and the workers at the quary mines complain about the exchange rate, since they are paid with NCR paper money. Basically, this is what F4 should have done. I'm okay with people still using caps, but at least, the writers must explain why.