Part of the problem I see is a major progressive generation gap at the top levels of elected government. Not even proper leftists, mind you, just progressives. Bernie Sanders is even older than Biden. The next oldest prominent name is AOC, who is almost 50 years younger and is only barely constitutionally eligible this year (by less than a month, even). There's probably someone political wonks can name in between those two, but they're not household names and would be unlikely to be able to spin up a campaign this quickly.

Decades of centrism have gutted the Democrats of anyone who could possibly stand up right now.