I think three hings have roasted Biden. He can't get crowds to show up, he keeps saying stupid things, he has a long record in the Senate of pandering to people or groups that were once important but are now considered as bad as nazis, and (a really big one) was a couple of weeks ago when, after he weathered the storm of being too touchy with women and young girls, he called a 10 year old good looking and put his hands all over her AGAIN.
That combo of stupidity, lack of juice, baggage, and inability to control his peverted side in public made the power bosses say "Nope, this guy will get mauled by Trump".
And possibly Hillary pulled some strings or let some dirt out. Because she's definitely running.