My pet theory still holds:
None of the candidates, besides Bernie, want the nomination - lest they get destroyed by Trump, wrecking future political career.
Bernie doesn’t expect to win, is ready to retire, and this is his last chance to get the biggest stage he’ll ever have.
In need of SOMEONE to play nominee, candidates are throwing their support behind Biden, who has no political future to lose. He’ll go thru the motions (himself, or via puppeteers), be lauded for a spirited attempt, and be shown the door (to a cognitive-care nursing home) November 4th.
And the biggest buyout. Plus he'll have created (with the help of academia and the press) a small army of malcontent fellow travelers.