I think this is a large part of it.
(https://www.brookings.edu/articles/voting-by-mail-in-a-pandemic-a-state-by-state-scorecard/)
States were awarded higher grades on this graphic for "Voters automatically receive a ballot" and "Ballot does not require a witness signature." They were docked points for "Voters are required to provide a copy of photo ID for the mail application and/or ballot."
I see a lot of left leaning As and Bs and a lot of right leaning Fs and Ds.
2020 Electoral College for comparison