Biden was against so many things that dems wanted, like single payer health care and medicare for all, and he directly enabled a far right genocide of innocents when 87% of dems (at one point anyway) did not. He also pretty openly sucked off the rich. He could have fixed our broken racist classist justice system and he went the wrong way entirely on that. He was against expanding the supreme court and now look where thats led. Straight to a sweep of every single lever of power and the stage set on his watch for very possibly the end of the republic.
Doing a bunch of unpopular things and then running again when he should have known he was both too unpopular and too medically incapabale to win essentially knifed every Dem right in the back.
I wouldnt bet on history being kind to Biden. I guess I could send you a much longer list of ways he was a terrible senator and a terrible president and deserved his low approvals, but whats it matter anymore?