Three possibilities:
1) Most programs have sent out their first batch of interview offers already. You may have not made the first cut, but youre on their wait list somewhere. You may get an offer later on if someone else decides to cancel and give up their spot.
2) Some programs are slow. November 15 is the latest deadline for application submissions for a number of programs, and conceivably they wanted to review as many applications as possible prior to extending interview offers.
3) Some programs are rude. You may never hear from them, one way or another.
Based on last year's interview thread people were getting interviews well into December.
At this stage of the game, there is no harm contacting the programs to get a status update. Be courteous. Hang in there.