But why did all the quitting candidates endorse him?
If Mayor Pete stayed in, he had no chance of winning but could keep enough delegates from Biden to ensure Bernie would win. This would pretty much end his career. Same thing with Kloubachar. So they did the smart thing and got out before Super Tuesday, and endorsed Biden.
Plus Mayor Pete has no political future in Indiana. It’s a red state only getting redder being mayor is the furtherest he can go. And he can’t remain mayor and run for president again; it just won’t look good. He is probably the best politician out of the bunch and realizes this. So his only way forward is either get a position in a Biden cabinet or carpet bag it to another state. Obviously a position with Biden if he wins is the easiest and most direct, and an endorsement does not cost him anything. Kloubacher probably secures the VP slot (judging by her recent gaffe this weekend), and let’s face it, she could very well end up president in that position if Biden won.
Now if Biden never ran, other more well known moderates would have, putting the Dems in a better place. This also assumes the other moderates would have campaigned better, not allowing Bernie to tack up votes and split the party.