Understandably, there are many people who are upset, as this mirrors the debacle with Fallout 3, where the fixed ending was removed with the "Broken Steel" expansion pack that allowed continued play.
For those who do not like to read into the things that they spend their (parents') paychecks on, I will help: Obsidian has made statements several times explaining that it was carefully considered and ultimately decided against because the ramifications of your actions in New Vegas would be biblical and thus, too grand to depict, even in downloadable content. Plus, there would be nothing left to do, really.