In states where a reason is required, people voting absentee are often elderly or military - which I’d think would skew red. In states where anyone can vote by Mail, the advantage is convenience. Preferring convenience seems like it wouldn’t be inherently a Democrat trait. Seems like it would be more evenly balanced. Concerns over votes being counted correctly might explain it, if Republicans have that much more distrust than Democrats. I would guess that counts for a portion of the disparity, but hard to believe it’s that wide of a gap