Meanwhile, the Legion campaign and the House campaign only have a few problems for you to fix. At most, maybe you'll have to fix a few errands for them, like helping Vulpes with his contacts in New Vegas and McCarran, but for the most part, they've got things under control. The NCR, however, even if you win Hoover Dam for them, it's the equivalent of fixing a broken-down ship with duct tape. They won the First Battle of Hoover Dam, and it only became worse for them, not better. What's to say that they won't squander the second victory that they get? Or even a third?
The problems with the NCR stem from its red-tape bureaucracy and its disorganized nature which invites infighting, which the Courier CANNOT fix. You can fix the problems in New Vegas by working for House or replacing him. You can fix the problems for the Legion by saving Caesar from his brain tumor and doing enough good deeds that he names you his successor. You cannot fix the NCR, even after winning New Vegas for them, since you do not fix the source of their problems. Even if you run for the NCR Presidency and win, chances are, if you try to "reform" too many things, the Brahmin Barons, landowners, and NCR political elite might bankroll a rebellion against you anyways for "rocking the boat", and they might even turn the NCR military against you.