Well, effectively, what happened is that the Democratic Party had a primary election, where people voted on who they wanted the Democratic nominee to be. The thing is, conventional political wisdom is that the incumbent president has a large advantage simply from being the incumbent - people who are uncertain tend to stick to the status quo, and name recognition is as high as one could wish for. So, in the primary, no one serious ran against him because:
A. It would have been a waste of time and money
and
B. Doing so would’ve marked one out as very much ‘not a team player’, so to speak.
Thus, he won the primary election. However, after the primaries, he had a disastrous debate performance, and several highly influential members of the Democratic Party not only called for him to step down, but alleged mental decline (not necessarily dementia, but a serious decline in Biden’s sharpness and energy which raised questions about his ability to run the kind of vigorous campaign needed), even just since January of this year. Biden resisted calls to step down at first, and since he was the winner of the primaries, he was the only one who could realistically make the choice. He eventually bowed to public and private pressure and stepped down.