The link above cites previously planned renovations to the Dean Dome as the reason why.
One thing is we could be getting ahead of ourselves. It is not clear, at this point, whether UNC would actually host an NIT game. According to the latest NIT-ology, UNC would be a #6 seed. According to the NIT selection procedures, the higher seed has the option to host games:
Higher-seeded teams will have the first option to host games as they advance, unless extenuating logistical circumstances (i.e., travel, lodging and/or facility availability or NCAA Championship guidelines) preclude such an opportunity.
On the flip side of that, it is possible the selection committee pushes UNC higher, especially if they can win a couple of ACC Tournament games or say, beat Duke this weekend. Whatever the case, UNC probably ranks as one of the best ratings draws the NIT has seen in quite some time.
If they do opt for Carmichael Auditorium for NIT games, they would be a lot of fun. It also would be very nostalgic for some folks who remember the good old days. Plus, UNC has been running about 5000 below capacity in the Dean Dome over the past few games. An NIT game might get a better crowd but if they can fill up Carmichael, it would make for a very loud environment.