In Fallout world canon, the western BoS has officially banished the BoS branch in MidWestern and the Branch in DC is in Civil war. So I would say the whole BoS is falling apart.
The courier left the vault by 19, by the time he got shot in the head in NV, he is about 29 or 30. Consider the average life expediency of people in a fallout world is just about 3x, after the battle of Hoover, he will probably want to settle down and have a life, or his luck finally ran out and get killed by a lead pipe.........And from my point of view, if anyone want to make a "DLC mod" between FO3 and FNV, there should be one about the lone wander help rebuilding the railway, help building a settlement (and met ulysis, and wooo a single mum who has a kid that play guitar), and become a courier......
The image below is a "map" of US by the time of FNV, be caution though, most location in Legion territory are from FOvB so they aren't canon.
But still, you can see NCR has so much immediately treat at their doorstep: They are fighting the Legion, the Super mutants in Maripsoa, their war with the BoS, raiders in the Baja panusila, and even those in Shi are a potential hot spot.....I wonder NCR can allocate any resources even to send an exploration team to DC.
Also, in Fallout Tactics, the mid west region is suffering from radioactive dust storm practically the whole year, so it is not that hospitable..... even the enclave in their prime will have a hard time maintaining the railway,,,,,,
