I see this come up a lot in discussions about voting in America. Postal votes disproportionately go to Democrats, hence the Democrats want to expand postal voting while Republicans want to restrict it (and insist there is totally a bunch of fraud going on).
I’ve googled with a few search engines and haven’t found a convincing reason. Lots of evidence that the skew is real, but no explanation as to why. Indeed, if one just looks at demographics, one would expect postal voting to benefit Republicans by facilitating votes from people in the countryside who live far away from voting centers.
So what actually gives?
Access to voting is the foundation of democracy. Sane systems try to minimize any “pressure” to not vote, for any reason, because any such pressure is very likely to hit some demographics harder than others. The Republicans in particular rather blatantly rely on weaponizing this as a way of subverting democratic principles, by making it disproportionately hard to vote if you’re working, or poor, or young, or a minority of pretty much any kind.
Therefore, anything that increases access to voting, and levels the playing field, is worse for the GOP than being able to keep up the status quo of voter suppression. Hence their extremely shrill opposition to mail, and also the (“hilarious”) claims of “fraud” - painting the picture of your democracy being subverted is a handy talking point while you’re busy subverting your democracy.
So the boring answer is that your question is sort of back-to-front: it’s not that the mail ballots are skewed as such, it’s that access to in-person voting is. Mail ballots favor the Democrats because it is their voter base that’s (in this case, anyway) being suppressed.