I understand that mail-in ballots would favor the democrats in any and all states, because of the conversation around it, which party was encouraging it, etc. But why would the advantage be so much larger, massively so, in those swing states vs. other states?
There may be a good explanation, but I haven't seen it so far.
As a PA person and with PA being on top of that list, I'll poke a little at factors in PA.
1. Obvious one first - Trump's denouncing and fear-mongering about mail-in voting, especially in PA, has dramatically more effect on Republican voters (who already have been seeped in frequently unquestioned false voter fraud claims for many years now). As for Democrats, various Democratic Party favoring groups have been pushing absentee voting hard and effectively since the dangers of COVID became more apparent as a good way to help keep everyone much safer. Such was already quite showing its effects during the primaries, let alone later on.
2. Slightly less obvious one - The location of where the people who used the absentee ballots is. Philadelphia, for example, as a general rule, is a very strongly Democratic Party stronghold, and a heck of a lot of the mail-in votes came there. The ratio was a bit lower in more Republican counties and there were far fewer.
To poke at that further, though,
Politico has some relevant information.
That means that 81 percent of state voters who were sent those ballots have returned them. To break it down, more than 1.6 million of those ballots were from registered Democrats, 586,000 were from Republicans, and 278,000 were from independents or third-party voters.
This isn’t surprising or necessarily revealing: Political insiders have expected for months that Democrats would vote disproportionately by mail, while Republicans would vote disproportionately in person.
One stat that is interesting, though, is that 84 percent of registered Democrats who have been sent mail-in or absentee ballots have returned them, compared with 74 percent of Republicans.
When all is said and done, Kathy Boockvar, Pennsylvania’s Secretary of the Commonwealth, said she expects to receive as many as 2.6 million mail-in and absentee ballots. A Philadelphia election official estimated that 400,000 such votes would come from the city.
It's also safe to say that Trump is much more toxic among the Republicans who were concerned enough about COVID to request absentee ballots in the first place than he is among the general population of Republicans.
3. Both of those are related to the Republican Party having made mail-in voting and COVID into partisan issues and leading the right-wing propaganda machine into spreading disinformation on both. To borrow some stats from
Bexar County, TX (which, yes, is not PA, so the numbers here are probably not exactly the same), because there's some indirect relevance - well above 80% of Democrats consider absentee ballots to be safe, easy, accurate, and secure, while only like 20% of Republicans consider absentee ballots to be safe, accurate, or secure... and only about half consider it to be easy. For that matter, less than half of Bexar Republicans think that it has lower health risks than in person voting

.