The main reasons I think it could be Hillary, not too long ago she indicated that she has entertained running again, and that she thinks she could beat Trump this time around.
During the Senate impeachment trial the House Managers brought up the Bidens at least 400 times, that according to Trump's Defense Team. If these Democrats were wanting him as their nominee, why draw so much negative attention towards him, which this would do, and did do, the fact Trump's Defense then raised these issues numerous times, in response to the House managers bringing the Bidens up that many times?
Then there is the deal with Bernie Sanders. Some of these Democrats don't want him ending up as the nominee either. If Joe Biden and Bernie Sanders were out of the picture for whatever reasons, who does that leave left for the Democrats to go head on with Trump? Bring Hillary Clinton back into the picture, maybe the Democrats think they now have someone that can actually beat Trump.