They made it able for you to be able for you to go on in fallout 3 why wont they do it in Fallout New Vegas.
Because New Vegas' gameplay is different (and closer to the original Fallout games) than Fallout 3's. Fo3 is loaded with inane dungeons and random mook mutants and raiders to kill. New Vegas' content comes overwhelmingly from quests. Most locations have a quest associated with them.
What this means is that once the quest is done, there's little reason to go back.
We can see why freeplay after the end would svck in this game by looking at two similar games which allow for it: Fallout 2 and Mass Effect 2. In both of these games, there is no real reason to continue playing after the end. In FO2 the whole freeplay mechanic was basically one huge easter egg, with VC and NR people commenting on your actions in the endgame. In ME2 the only reason to play after the end is to play the Shadowbroker DLC.
Like these two games, New Vegas lacks the mindless mobs necessary to make freeplay after the end worthwhile like in Oblivion or Fallout 3.