One alternate theory (although even I admit this is relatively implausible):
The elephant in the room is that the Clintons will go scorched earth against Biden as soon as he enters the race, thus it is his best interest to enter the race as soon before the race gears up as possible. A December entry allows him to stay on the
overwhelming majority of ballots (except for New Hampshire, Florida, and a few Deep Southern primaries that Hillary will win in a landslide anyway), plus he would dominate the news cycle just before the Christmas lull. He would only be a walking target for the month of January, which is a risk he will have to take.
