Because of anything he's accomplished.
What happens when a party panders to a lost case?
You open up this seismic and intractable gap between the young and old.
No candidate was going to bridge that gap
But older always vote
And younger people don't, and have more road blocks to do so when they try.

The finger should be firmly pointed at the Democratic party for this divide where either a Biden candidacy or a Sanders candidacy doesn't represent the wishes of a sizeable portion of the party.