Biden isn't earning it.
He's positioned himself as entitled to the presidency as Obama's Veep. He's tried to wear the mantle of the inevitable candidate. Neither of those is appealing.
He's also suggested he'll be a compromiser, when others are defining policy ideas with passion, creating the mandate they will need for action later on. I don't think Dems are looking for compromise. They don't see a Republican Party with ideas you can compromise with. They want a party that will forge its own path in the direction its voters wish to go. They hope for a president who can deliver on the ideas he or she defines. Or at least who can undo the damage done by Trump.
Biden's a decent guy, a useful senator, but I still don't see him as a president. Better than Trump, obviously. But there are candidates with more to offer at the moment. With a toughness about them that he seems to lack. And the vision to go with it.