Becuase unlike Fallout 3, New Vegas actually had choice in the endings (Four major endings plus loads of different variations on minor quests and storylines)
Would have taken too much time and effort on Obsidians part to have created a post-ending world that could reflect all of your choices.
And Broken Steel fixed nothing. It broke the levelling system that it tried to balance out with bullet sponges, gave us a selection of rediculous cheat perks and gave us a really stupid 'final mission' ie: Collect an endless supply of cameras and batteries. It railroaded us even further into helping the BOS (Something we'd been forced to do in the main story) and invalidated our heroic sacrifice at the end of the main story so that we could become little more than an errand boy/girl.
I can't say it didn't do that. But the level cap of 20 was way too low, and Broken Steel actually added in a ton of true END GAME gameplay.
Considering FO3 was not really a FO game as much as it was "oblivion with guns", the open free roam of BS actually "Fixed" that game.
FONV is more of a true FO game.