This can only be explained by Biden's slow-motion implosion. My belief in that Warren could not be the Democrats' nominee is because she's a stone-cold Hillary 2.0 loser to Trump:
From the beginning of this mess, I’d never imagined that Donald Trump could be lucky enough to draw Warren as an opponent, but at this point, it’s impossible to deny that she looks like she has a real chance. That’s kind of amazing because she really is a dud of a candidate. She’s terrible on the stump and leaves people with the impression that she’s lecturing them more than relating to them. And she’s already gone on the record with a bunch of policies that will ruin people’s financial situations while driving the country in an extreme leftward tilt.
If Trump is running even a modestly competent campaign next year he should be able to tear her to shreds.
Since it's confession time, I was also very wrong about Kamala Harris. Sweet mercy, she's a terrible candidate.