There's no mystery or conspiracy about it -- it was just there were fewer excuses to NOT vote.
Postal voting has indeed made it easier, but I think there is more to it than that: that would apply to both sides, except Trump went out of his way to diss postal voting and some Republicans may have taken this as an instruction not to use a postal vote.
I think a lot more people who voted Democrat in the Presidential race (but not always on the rest of the ballot: there were places in which the only Democrat who won was Biden) voted against Trump than voted for Biden, in reality.
What I mean is, if Biden had been up against a normal Republican candidate, he would have lost because people who are vaguely Democrat or have more-or-less Democrat sympathies wouldn't have bothered. And no Republican or Republican sympathiser would have voted for him, as some seem to have done in this election.
These same people were galvanised by the prospect of Trump winning into turning out in their hundreds of thousands, because four more years of Trump looked to them like a really horrible option.
Oh, and a largish tranche of the military, not a body usually noted for their Democrat sympathies, seem to have voted against Trump for all sorts of reasons: they didn't like being deprived of their houses and education facilities so the budget intended for them could be given to Trump's buddies to build a few miles of border wall, they didn't like the prospect of being illegally ordered to attack American civilians on American soil, they didn't like him insulting or slighting their war-dead, they really didn't like his attacks on McCain whom they saw as one of their own, they didn't like him calling them losers, and they just generally found him to be all mouth and no trousers about his much-vaunted "support" for the military. Particularly since they knew he was a coward who dodged the draft on spurious excuses.
So there was a record turnout for Biden, which would have been a record turnout for a blue-based baboon if that had been the alternative to Trump. Or as someone else said, a potato.
This does not answer the OP's question. I don't feel that I know why anyone voted for Trump, apart from feeling they will have been (for some reason incomprehensible to me) convinced they should do so and being very resistant to the idea of changing their mind about it after they said they would or they did so last time. Isn't that, along with "I always vote Republican/Democrat/Green" the reason for an awful lot of votes?