That might explain why democrats didn’t do better, but not why polling and media expected them to in the first place.
If the question is why democrats didn’t do better considering how awful trump and trumpism is, I think it’s partly effective branding by republicans tying all democrats to the extreme progressive wing. In Republican telling, all democrats are antifa supporting, police defunding aoc socialists. The electorate is also so polarized that most people just vote for their team. There are few undecided voters and few voters who would consider voting for the other team.