Because he is polarizing. There are people who really like him and therefore want to vote for him, and people who really hate him and therefore want to vote against him. And the polarization got worse from 2016 to 2020.
Additionally, because of Covid, a bunch of states allowed vote by mail. That did make it easier to commit fraud, but it also made it easier for people to cast legitimate votes. So, we got a lot more total votes.
“Because he is polarizing.”
O.K. Fair enough
“And the polarization got worse from 2016 to 2020.”
And that’s where your premise faulters. Trump became less polarizing between 2016 and 2020 because he had a record of sucess that he didn’t have in 2016. He gained support among independents, and in particular Blacks and Latinos between 2016 and 2020.