You can enter all programmed parts of N's castle in B/W/B2/W2. There are only 5 floors in N's Castle. In B/W, you cannot revisit N's Castle. Source
There is no point for the character or the plot to go into such places. Your character will only got to where the fight is. As for B2/W2, the other parts are unnecessary, and probably ruined/more unsafe then the main compartment, making no use to explore them. You probably could if you had free will, but the dev's code contains you. Game Freak Logic. You get used to it. :P
-Myself, Comment. Basically, Game Freak Logic. They didn't code it in is the only reason it isn't in there. There's no point to make a DS cartridge have a large, unused area on it when it already has to have 500+ Pokemon and 600-ish moves on it. The Developers were lazy and decided the area was purposeless and not worth the effort of coding it in.
