some if this is pretty predictable, though, isn't it?
after 3+ years of chaos and extremism, people crave calm and normalcy (not extremism in the opposite direction).
enthusiasm for bernie is clustered geographically and demographically, and he hasn't demonstrated an ability to broaden that base.
youth rallies are awesome but it's been proven time and again that young people don't actually show up and vote in meaningful numbers, while the most reliable voters are older one who typically lean moderate.
African Americans, particularly women, are the core of the democratic base. Bernie has never done well in that demo, and they largely have positive views of Biden from his time with Obama.
the other sizable portion of the democratic base is white suburban women, who again skew moderate and have never warmed to strong progressive positions.
I said upthread I had no idea which dem I'd vote for, they were all either weak or unelectable. Bernie could more than hold his own on the debate stage versus trump...in fact, my biggest fear is he gets so worked up that he has another heart attack! Biden is a really weak choice who could get creamed in a one-on-one debate, but a lot of it just depends on how fed up people are with the trump chaos.