Well there was a lot of people, especially on the dem side of the spectrum, who weren't a fan of his public unwavering support of the genocide of the Palestinian people. Internal investigations within the party allegedly agree with this analysis.

But yes, people dislike Biden for a lot of things he didn't deserve to be disliked for, such as inflation which was caused by COVID and Trump and which Biden did a fantastic job of controlling, but which parts of the public simply perceived as "inflation went wild under Biden". Still, even though it's not fair, the message "I will do nothing different from (unpopular incumbent)" isn't great campaign strategy, in my opinion.