The Ds have to have Biden win because they can sacrifice him. he is already a loser and hussein didn't endorse him..no great loss
The Ds cannot afford to have Bernie as the nominee because Bernie espouses everything they believe in. When he loses to Trump, thei entire progressive system loses big.
So why are the others running? Because they all know they will lose to Trump.
I think the rest are running just to take down Bernie.
Pete is running to get some creds for being the first gay president, assuming nobody counts hussein.