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?
Regarding rigging voting machines, that’s conspiracy theory territory, however you did miss that Democrats are more likely to be working jobs where you can’t easily take time off to vote, or it’s that or lunch, or the time they give you isn’t enough, so doing it in advance by post is pretty much the only way you can vote.