Is it coincidence that the B-2 and YB-49 flying wings have exactly the same wingspan?
A random guess without any supporting information. (please edit or delete if this turns out to be inaccurate)

I would guess the logistics of manufacturing. It could the space needed for handling the aircraft in and/or out of manufacturing buildings.
The B-2 program was wildly expensive due to the large number of new technologies involved.

So I guess it was a prudent, maybe even progam-saving decision, to use existing manufacturing facilities are far as possible.