No Christmas Tree?

After sitting in my inbox for about 3 weeks, I finally watched a video that John Piper and his family had posted for Christmas. One thing that I noticed is that they did not have a Christmas tree in their house. Now, granted, maybe the tree is off camera. But I have wondered this for a while. Why do we put up a Christmas tree around Christmas? Serious question. Anyone know how the tradition began?

Also, here is the video that I am referring to.