He was always - from the time he joined the primary process - a card board cut out, promoted because being a former vice president gave the appearance of being more electable than all the other B-grade candidates from his party.
Obama and the left wanted Harris or Warren, because of the political bent and because of their sex most of all.
When every did so poorly in the early primaries, Obama and the Left got behind Biden without big announcements about it. Those on their big donor lists were told to give to and support Biden. While even then they knew he was getting pre-dementia. All the better, as long as they could use him as card board cut out that they wanted. Their Vice President they imposed on Biden would be President soon enough. Meanwhile the heavy lifting would be done through Pelosi and Schumer in Congress, not the White House. Biden could continue as their front man as long as possible.