I was talking with my wife in the car earlier today that I suspect coronavirus is helping Biden and hurting both Trump and Bernie big time, as people are now looking for comfort, how things were, normalcy, etc., etc., and Bernie's "revolution" mantra sounds scary to those with coronavirus concerns, and sure enough, those who say it is a big concern, according to NPR, are voting for Biden over Bernie by over 2:1 numbers. Tough break for Bernie. Especially because national health are and sick leave and free vaccines (Bernie's policy) would do way more to stop it.
It's the US. Simpler things would explain why anyone would choose Biden. Some people are just stuck with him being the VP of Obama, as if he did anything.
Also, old people like him. From minority groups, he only has an edge with black people, while massively losing in the latino community. Some other black people support Bernie, like actual political ones:
From a telling comment there: "Jesse Jackson endorsed Bernie because Bernie stood with him before, while Kamala, Beto and Pete endorse Biden after saying he’s not fit to be president."