Extremely important point. The entire argument I have been hearing why Sanders has a better shot beating Trump than Biden is because of the large turn out he inspires.I don't really see it. I mean, he got a bunch of endorsements in the past few days, but candidates drop out every primary and endorse another candidate. There's nothing unusual there. It makes sense to time your endorsements for the time they'd have the most effect.
The fact is it appears Sanders hasn't been able to achieve the increase in turn out and he will likely lose the primary because of that. The DNC presumably want Biden, of course they do, he's a Democrat, but I don't see what has happened that has been so unfair to Sanders.
Ultimately the Sanders campaign gambled on turn out and gambled that more candidates would stay in so they could win with a low share of the vote - that hasn't happened and he appears to have a ceiling. The blame, if there is any, is on the Sanders campaigns gambles appearing to have failed. And with that said things can still change quickly, like they did for Biden.
Guess what, Biden is inspiring a larger turnout. Kids actually prefer arguing in an internet forum how awesome Sanders is, but not go and wait in a queue to vote. Boomers instead wait for hours and cast votes. They are a much reliable source of votes.