Probably due to it being post Christmas and the fact non of the rides are operational at these temps.
Post Christmas isn't the problem. I've seen incredibly busy days after Christmas. Date wise for many it's a big time for them to go because of vacations, and when the weather is decent, they will have huge days during this week.
It's the cold, pure and simple. People don't want to be out in that cold for long periods of time, so the don't go. It's miserable in the park when it's that frigid.
The ride closures probably don't really make much of a difference as it seems guests are oblivious to the fact that the rides are closed because of temperature until after they get there and are in the park. There's not a lot of notice outside about the closures, and even if there were I doubt people would see them or pay any attention. When I worked there I had a ton of guests confused and surprised about the rides being closed because of the cold.
Really, this week is so dependent on the weather. If the weather is nice, it can easily be the biggest week of the year. But when it's frigid cold like this, it won't be. A little cold air won't keep them away, but this much will. I'll bet New Year's Eve is a bomb when they're predicting the high temperatures won't get out of the teens.