By endorsing Clinton, Sanders would instantly and irretrievably lose the support of 99% of his present supporters, and he knows it.
Because they don’t support Sanders for who he is. They support him because he’s the only presently available vehicle for conscientious objectors to the Third Way takeover of the Democratic Party.
He cannot bring votes to Hillary - he knows it, she knows it, Biden and the rest all know it. The only thing his endorsement would represent is the subjugation of the faction that supports him.
In case anyone needs proof, check out how Sanders supporters instantly turned on Elizabeth Warren the moment she endorsed Clinton: http://jackpineradicals.com/boards/topic/ouch/