I think their reason for not allowing you to play after the main quest is that too much would change. Given that there are three different routes you could have gone. ie NCR, Legion, or yourself. I can see how that might make things a bit difficult to making NV seem like the final battle actually did somthing and was not just a WWI run at the trenches then run back with no real gain or loss. Also, NV might seem kind of empty and void of life without one or more of the factions running around. I personally think that the devs could have done somthing to allow you to play afterwards and not have it such a massive change as to make it unecessarily complex and impossible...but I'm not a modder or a dev so I don't really know.
I think theres a way it could have been handled, just need to look outside of the box. They could have set it up where if you side with House, Cesar ends up getting wiped out you will see a few Cesar loyalists around just like the Enclave in FO3. You could see limited numbers of NCR troops as well and see more of houses secuirtrons. Only enemy that would be around would be the few remaining NCR and Cesar forces and the typical enemies such as the fiends.
I do understand why the cut the game off though but I feel with alittle time and thought put into it there could be a way to keep the game afterwards without devoiding the wasteland of enemies or people.