Admittedly, when I talk to less politically minded people about it there are basically three camps of anti-biden libs (or at least hesistant), and until recently mainly just the first two.

• People who see that the world/their lives aren't getting better and don't like that. Tends to be poorer, less media-watching. They also tend to note how obviously senile he is.
• People who (presumably based on half remembered CNN talking points and newspaper headlines), think he's going too far left. (tends to be more middle class and up)
• People who can't stomach voting for him because of Palestine, more recently, or previously other things like his racist actions on crime/busing, or SA allegations, or what have you.

I don't think Palestine is the top top issue for likely voters, but it should definitely be in a list of the top 7 things killing biden's chances rn. With young people it might be up there as #1 or #2 but with the demos that voot the hardest less so. The narrative that he's going too far to the left is manufactured whole cloth by the media, but its been pretty successful, it's taken root for anyone who pays attention to MSM, though it hasn't gotten to the level of being accepted as baseline fact yet, more of just a vibe.