Well, because it wouldn't be Fallout it would be some Area 51 game.
Fallout isn't about Aliens it's about how the survivors of a Nuclear War would survive and try and rebuild some form of civilization. The Alien random encounter from Fallout 1 was just supposed to be a nod to the Roswell craze of the 50's and also a way to give you a unique gun that you wouldn't get anywhere else.
Fallout 3 included it's own similar random encounter, unfortunately being the "OMG it would be soooo cool to make that a DLC!11 lolz!!11!" is the attitude of the Mothership Zeta crew. It never should have been made, they wasted time on content that frankly turned Fallout 3 into a crappy 'Destroy all Humans' retarded cousin. I've played M.Z. and I'm really glade I didn't pay for it.
Now I understand you are a Fallout 3 Fan and that you enjoy it and would love more content similar to what Fallout 3 is, and yes I enjoyed Fallout 3 as well but I was a Fallout fan back in the days of Fallout 1 & 2. This gives me a fuller view of the Fallout universe and I can tell you that the alien encounter was never meant to be more than the random encounter in Fallout 1.
Now go ahead and tell me that I'm being stubborn or whatever or that the franchise is in new hands so it's up to them to say what is or isn't cannon. Well that may be true but at least as far as New Vegas is concerned the Fallout 1 & 2 guys (or some of them) are going to stick closer to the Fallout 1 & 2 style rather than F3 and for me that is going to make it a better game with out having even played it.
One final note, I'm not saying that you are wrong and I'm right, but as far as I'm concerned nothing that Bethsoft, or for that matter Obsidian does to the franchise can overwrite what is set as cannon in the first two games, and even though the M.Z. DLC doesn't strictly change anything set up in Fallout 1 or 2 but it does go against a standard setup by the first two games.
Finally, I just don't agree that a town of aliens would make for a good addition to a Fallout game, and there are plenty of aliens in plenty of games, but Fallout is not the right franchise for that in my opinion. You know though the best part is you can have you opinion and I can have mine and one of us my not be happy with or without certain content but I'm sure that what they do put in the game will more than make up for it.
...so that is how I see it.
"Working To Build A Better Wasteland!"
Whenever I'm in a tough situation, I just ask myself, "What would Altra do?"--------------------
~R.I.P Fan Made Fallout 2003-2009~