A few years ago some hard-working soul put together a good summary of why the National is where it is and isn't where it isn't. (Too wiped out now to plow through the archives to look for the thread.)
Essentially it came down to three things:
availability of floor space--the venue has to be big enough, but not too big.
scheduling flexibility--some venues won't schedule events out far enough in advance, and others want committments too far in the future
union labor--driving right up to your space ain't gonna happen in a place like Boston or NYC. Only union labor can handle unloading and loading at some venues.
Bill