They may not know either.

I suspect a production/quality problem somewhere either at Apple or a supplier threw things for a loop. If it has been resolved, they can answer your question. If it's still being worked, they are a lot more frustrated than we are.
My guess is that this problem surfaced in late summer. I suspect they had planned to release the nMP in early November after the Oct 22 announcement. By the time Oct 22 came around, they realized Nov. wasn't going to work and they crossed their fingers and said Dec. This is pure speculation, but I've been inside a variety of software projects that slipped their deadlines and I've been close to some military hardware project that had similar problems. When a date slips there is a great tendency to be very optimistic in setting the new date. e.g. Affordable Health Care Act Web Site