Why, you ask? Because there absolutely nothing to lose, and everything to win doing so. Harris is _good_ at that. She's also a former prosecutor, readily capable of tearing a convicted felon to pieces. And she needs to up visibility. Either she needs to be a very prominent VP, or she needs to all of a sudden be a credible top-of-the-ticlet candidate.
And Biden? Biden is good at talking about positive things - far less so attacking, or responding to attacks.