Biden's campaign was running on fumes last week. No money, no presence on the ground in CA or many other large ST states. And no real vision of why he's running to inspire voters, other than a claim to be able to unseat Trump. Much like Clinton in 2016.
What happened was the DNC got alarmed at the prospect of Sanders building an unsurmountable lead yesterday, based on performance in NV and polling afterwards, and engineered the other moderates dropping out. Because Sanders threatens the DNC, and their cozy shadow-RNC insider shell game of pretending to represent their base and its economic priorities, but prefering to court rich donors and corporations in order to get rich themselves and keep access to power. This devils bargain requires them to neuter anything that threatens the status quo, and Biden is the perfect totem for this. He has that track record. He reassures rich donors. He did reassure them last year - he told them he wouldn't change anything fundamentally.
The cable and broadcast media support this: they are corporations too and are threatened by Sander's assault on healthcare profiteering - most of the ads that run on their shows are for prescription drugs. MSNBC have openly attacked Sanders, and CNN do it more discretely. Their pundit panels are stacked with DNC operators, veterans of Clinton and Obama campaigns and those ambitious enough to play the insider game and not bite the hands that feed them by calling attention to the dysfunction in the capitol. All of these people are there to reassure their audience that the adults are in charge, that the economy is doing well (because unemployment is low) and that wars are patriotic.
The message being amplified over and over again is that Sanders is not "us" (he isn't), he's a threat (he is to them), and he must be stopped, and the DNC needs to stop him. This has been going on for weeks now. Given that the voters broke massively for Biden last night, and it wasn't because anyone for him knocked on their doors or called them up or they saw any of his ads, it's likely that this drumbeat TV messaging and the big win in SC swayed them. The key endorsement was Clyburn, and he's deep in the pockets of the pharma industry. He took over a million in funding from them over 10 years. He's another symbol of what's wrong with the DNC. Of course he's not going to endorse the candidate angry about his constituents (and everyone else) being ambushed and bankrupted by medical bills.
This isn't entirely a CT narrative, and that Democrat voters are no more than just zombies with transmitters, but I think the reality is more complex than, well, Sanders wasn't able to make his case. He's been right about just about every negative trend in the past 20-30 years, and this Coronavirus outbreak and the threat it poses underlines every point he's making about the need for access to medical care at the point of delivery, paid sick leave, and the compassionate treatment of the most impoverished members of society. His candidacy is not without risks, but progressives have no seat at the table, that much is obvious, and yet again we're being asked to back a weak, compromised, corrupt, establishment-friendly candidate, who doesn't inspire anyone, just attracts those willing to settle, and not willing to think about what helped create Trump in the first place.