The same way everyone else does. "The party apparatus" is no one's to command until there is a nominee.
Technically it's true. In fact we all know how it is. The same applied to Trump's campaign, his party and mainstream media. Clinton/Biden was/is the establishment choice.
Biden had NO money. He had NO staff. He was being left for dead by the media. And he just went out and got votes, including the very important votes of Amy Klobacher and Pete Buttigieg and Beto O'Roarke and James Clyburn. That's campaigning.
No, that's being a safer bet generally and especially not as strange to Black Democratic voters (which made substantial part of Super Tuesday's votes cast in southern states) who tend to be socially conservative, more family-value-welfare voters, as typically New England's and Western' academia and liberal favourite. Sanders is also simply a little awkward, even to Biden.
Word from outside US: Bloomberg's was no Democratic Trump, Sander's is. There's more to value in candidate than to disregard him because of his sexist, business-world background. His views on foreign and economic policy should matter. But it's no surprise to me, that Democrats don't get it.
And there are as many similraties on Sanders-Trump line as differences. Including being a real outsider, clearly caring about his country and its prospects, knowledge that America is being ripped off both by neocons, knwoing that neocons administration were naive and bleeding America financially, while sleeping when it comes to the real threat (PRC), knowing that fiscal situations and debt matter, more than who grabbed who by what. You may not like candidate, I don;t like Trump personally and he speaks like someone illiterate, but I would vote for him in a heartbeat over Clinton/Biden because he knows where real geopolitcal, economic, and technological challenge is (in the Pacific). And national security, commerce, diplomacy, and enabling small enterprises and avarage workers to generate more wealth, should take precedent to adjusting how affluent citizens are (which is healthcare debate, or other cultural wars like equal right amendment) when they are loosing world's stability for nation that is not that opulent, but disciplined and devoted (China). America cannot afford that and world needs her.
Where the real power rests currently in Washington is another topic (Trump or Pence or others). But current administration is setting right track for what to focused at. China
Sanders and Trump economic thinking is too simplified to be taken real, but they both know rampant inequalities, unrestricted free market for corporations (which both Dems and GOP enabled) and lack of public infrastructure investments are problems of America, and adressed and steered towards benefit in China. Trump saw it with Japan, with China.