Well maybe not, but it still just don't really make any sence. How come DC is still in ruins, how come every city is in ruins and so on almost? Why only rebuild new vegas when u can conquer the whole planet.
Well for one, there's the matter of resources and supplies. It's a lot harder to send an entire (large) military force, across such a large land mass, than it is to just send a small "caravan" like force. You'd have to feed a lot more people, and your supply lines would be stretched quite thin, if you did send such a large force.
Secondly, like Kjarista said. Security, is something D.C. just didn't have. The West Coast, is much more civilized (technology wise) and collected, than on the East Coast. The East Coast, is still like it was 100 years after the bombs fell, or even 50 years before. They just had no major organization, that could create something like the NCR. And before you say, "what about the Enclave?" The Enclave didn't exist on the East Coast until Eden called them to go east and regroup.
Third, Arroyo, and other places from FO1-2 have probably been upgraded last we saw of them, and are probably no longer in such ruins like New Vegas.