Putting Bernie in would help Democrats in an area where they're weakest, so they'd never do that.
Realistically, I'm pretty sure even Bernie has said he's too old to be president (could have been propaganda). Not a lot better, but i'd still probably vote for Elizabeth Warren if she was on the ticket. I did always feel like a lot of her ideas were either piggybacked from Bernie or kind of lacking in substance but that could be personal bias somewhere on my part or just the way the media portrayed her. Even then, it did always feel like she was headed to a good place.
Warren isn't an option either though, the real reason is that the party wants an establishment candidate that won't jeopardize the way things are run. They'll never let anyone progressive run on the ticket again.
Group 1 unfortunately has a tremendous amount of overlap with groups who typically are unlikely to vote, I think it's why the concern there hasn't even escalated beyond stern words when the cameras are on. Some of the strongest swing groups also tend to be a bit biased against Harris, I think that's why they're hesitant to choose her.
tl;dr: Democrats have decided that young people are going to continue to find a reason to not vote and swing voters trend towards being at least mildly racist/sexist old people.