It's because next Sunday is Septuagesima -- hard to celebrate and fast at the same time...
Many years you CAN celebrate until Feb 2, but I feel compelled to take down my tree by next Sunday, as it's pre-Lent. Some people begin fasting at that point -- but regardless, the Church begins her mourning at Septuagesima (no more Alleluia, etc.)
Overall, I like to be in line with the spirit of the Church.
Matthew