Sanders simply didn't have the numbers. And wouldn't have had the numbers, even if the DNC hadn't actively been backing Clinton and then Biden.
The 'problem' with Sanders is that he's a pretty solid democratic socialist. That's scary for a lot of centrist voters in a way that Trump isn't, or Mittens Romney wasn't. Is that dumb? Yes. But that's still the way it is. You can be angry about the Overton window moving to the right, but it has moved, and when you're looking at electability of candidates, you need to pay attention to it. You need to move that window left before the elections, and that requires a ground game that Dems simply don't have yet. (And may never have, TBH.)
Republicans have been moving that Overton window to the right for literally decades. The first concerted effort that I can remember was with Newt Gingritch and his "Contract With America", and then the Tea Party. We haven't had that kind of concerted effort from people on the left in the US, and we need it if we ever expect a candidate like Sanders to be viable.
(And really, Elizabeth Warren has nearly identical politics, but is less well-known as being on the left. She'd probably be a more solid choice at this point to capture centrists.)
