why wouldn't you want it?
Because Freeplay is pointless in a game where the draw is quests and writing, rather than grinding.
Case in point: Fallout 2, Mass Effect 2, and Fallout 3. The first two examples were strongly story driven but also have freeplay after the end. After you beat "the game", there is little impetus to keep playing. A few N7 missions you didn't do beforehand, or listening to drugged up Reno hokers talk about how you blew the oil rig isn't much fun after maybe ten minutes.
Fallout 3, meanwhile, eventually allowed FP. And there, it worked. Because the primary draw of FO3 was sandbox exploration with a bunch of dungeons and exploding raiders/super mutants/Enclave/ghouls, rather than story and writing.
New Vegas is not Fallout 3. New Vegas' strength is its writing and choices, much like its predecessors. FP after the end is nearly worthless here because the major draw is how the quests interact with the ending. NV has almost no pure dungeons; most locations have quests associated with them.
What, do you really want to go and shoot another five fiends in the head? Really? What's the point? It's not like Fo3 where there was a great variety in enemy-held locations (since most locations in the game were dungeons rather than more meaningful places.) New Vegas simply isn't a huge sandbox to play in; there's not enough sand (dungeons) to build anything. There's nothing inherently wrong with that, since NV's strenghts come from its writing and choices and replayability. Note that replayability means reasons to start a new game, not "reasons to continue one character for 9001 hours"
The only way Freeplay would even work from a purely gameplay standpoint and ignoring how it cheapens the endings by making your actions meaningless is if it included a dungeon pack with a bunch of locations filled with loot and enemies.