It’s nothing particularly specific. Conservatives hate him because they hate all Democrats. Leftists hate him because he’s a milquetoast liberal at best. In fact, I was surprised by some of his policies being more progressive than expected, but he still comes with a long history that suggests otherwise.

The people who actually like him are middle of the road Democrats. There aren’t enough of those to make a solid base of support. Biden’s victory hinges on having enough people further to the left who know what a disaster another Trump Administration would be and vote for Biden despite misgivings.