Loose rivets - the NG has poor pod clearance (better than a jumbo though) and will strike a pod at 15° in the landing attitude.
Boeing have obviously tested these conditions and the have decided the margin against podstrike is not enough in those wind conditions. The massively over -developed NG is assuredly not an early jet transport and if one had a pod strike ignoring the FCTM advice a meeting would surely follow even at more enlightened airlines.