I believe Trump would have won 2020 had the COVID pandemic not happened. Things were very chaotic in 2020 America. Biden and his extensive experience in the federal government looked reassuring to a lot of Americans. Biden would have had a tougher time against Trump had 2020 been more like 2019. I believe Biden would have had a tougher time against Bernie Sanders in the primaries had COVID not happened, though a counterargument is that Super Tuesday happened on March 3, before shelter-in-place policies were in effect in California.

A big reason for Trump's success despite his polarizing nature is the polarizing effects of the platforms of our two parties, which distinguish themselves on "culture war" issues such as abortion, gun rights, immigration, LGBT+ rights, and race relations. There are many Americans who love the MAGA agenda, and there are also many Americans who are not in 100% agreement with MAGA but who'd never vote for a Democrat since they feel that a candidate with the opposite cultural views is anathema. If third parties were more viable in America, the latter group of voters could vote for a candidate that is more to their temperament instead of voting for whomever the GOP nominee is.

Had COVID not happened, Trump might not have gone batshit crazy with a vendetta against the entire concept of independent federal agencies. Actively rejecting the advice coming from Fauci et al would seem to be a large part of what sensitized him to the larger pattern rather than just writing each instance off as an interpersonal issue.

(by "Trump" and "him" I mean the person himself plus his symbiotic ecosystem of enablers and followers)