More than his obvious slipping, due to the fact that he's the wrong color.
Most you drastically underestimate the degree to which the stuff I rail against has taken hold not only on the Left, but within the Democratic Party. There are all sorts of ways a party can internally sabotage a primary candidate they don't like. It didn't work with the GOP and Trump, but we've seen it work before and I think we saw it in 2016 with the Dems and Bernie.
By purging moderate candidates and officials, the party is now squarely in the camp of equality of results and an old white guy isn't a desired result. Like many other ideologues, they care more about purity than winning. Besides, they delude themselves into believing that if they just get a pure enough leftist, the American people will embrace him/her because, after all, previous Democratic candidates were merely "fake progressives." The "No True Scotsman Fallacy."
Link: https://www.politico.com/story/2019/07/29/top-dccc-staffer-out-amid-diversity-uproar-1439525