Because I believe (now moreso ever with the coronavirus rampant) that we need more leftwing influence in politics, and Biden is the opposite of that. I know it comes down to electability, but the safe candidate was played in 2016, in my opinion. If people think repeating that is going to help, fine. I just don't think it will. I also genuinely believe Sanders will struggle to run in four more years time, and there's nobody else at the level who could pick up that kind of a policy platform. Maybe Warren?
We're heading into territory which is exposing the flaws of "austerity" politics, of "trickle-down" economics, and everything inbetween, at a rate and scale faster than I even thought possible (again: the virus - not helping. But structurally, before that also). Obama could've put his weight behind Sanders. A ridiculous idea perhaps, especially given his political past with Biden, but he could've done. A lot of votes are won and lost on messaging (and a bunch aren't, I get that - some people will vote for the safe option regardless), and I've seen way too much stigma around anything left-of-centre in the past four years alone - in both the US and the UK - to have time for it anymore. Additionally, the Democrats could've spent four years changing the message, however slightly, on more leftist (for America, which isn't saying much) policy. They didn't. The best we got were people like AOC (who are excellent, but also singled out for abuse and ridicule, in addition to being junior members of the party - without much actual support from mainstream elements of the party).
Labour in the UK at least tried this, and fought an openly-hostile press (amongst whatever mistakes they made themselves) as well as opposing parties. I guess I just wish the (mainstream) Democrats had remotely that kind of backbone. I don't see them beating Trump at this game the Republicans have gotten down pat. Maybe they wouldn't have with Sanders either. We're at the point now where it's pretty much "wait and see".