Did you guys even look at the pictures of ground zero? I'm sorry but I think you are giving too much praise to human ingenuity. Someplace that small being hit that hard is not going to fair very well. If you are struggling to stablilize yourself and find food you aren't going to worry about rebuilding around you especially if so many things and people are trying to kill you.
That excuse only goes so far. It hasn't been only a few decades, it's been two centuries which is a
long time. The people of Megaton haven't really tried to do anything; they haven't established any sort of militia, they haven't built up any sort of governing body. They've had more than enough time given how complacent they are during Fallout 3; there's not a single sign that they are having difficulty surviving, quite the contrary actually. Even Rivet City has shown more initiative than Megaton, and Rivet City is only a few decades old.
megaton could probably not expand that much because of the way it was built and if you get too big it goes to anarchy (NCR).
The NCR itself isn't in a state of anarchy. The NCR is actually very stable, its problem is that it's trying to expand at too fast of a rate despite its conflicts with other groups, so when it comes to expansion into territories like New Vegas it's a bit harder to maintain control. Ambition comes at a price, and for the NCR they're stretching their forces thin in an attempt to control more than they are realistically able to control at the time. They are not falling into anarchy though, not even close; they're just having difficulty expanding further.
Megaton needs to actually build a real governing body and a military force before they can even think about expanding though, so at this point that's inconsequential.
The point of this thread is not what is wrong with the Capital Wasteland however; it is what is supposedly wrong with New Vegas. Personally I don't see a problem.