In my experience, students tend to drastically over-estimate what can be accomplished over the winter break. I remember friends who always took off with plans to read every chapter of their upcoming courses, work through a mountain of problems to get a jump start on the coming term, and then first day back it was rare that any of us had even opened a book!
And there are many reasons this happens. The biggest one, I think, is that most people need a break. University is hard work. Taking time to socialize, catch up on sleep, read things that are not assigned to you, and generally do some fun stuff is important to maintaining balance in your life. Healthy down time is what allows you to push yourself through those weeks where you have a mid-term every day and three assignments due, etc.
The other big one is that there tends to be a lot of extra-curricular stuff going on at this time - holiday parties, end-of-exam parties, family gatherings, old friends who want to get together, shopping for gifts... this all takes time. In some cases a single family get-together can be quite draining.
This is also a time for catching up on other life-responsibilities. This is the time to buy new clothes, move into a new apartment, go see your dentist, do your car maintenance, etc. - basically catch up on all the little things that you've been putting off while at school.