It's almost as if a growing population leads to a growing number of voters and thus a growing number of votes.
As now, Trump railed against mail-in voting in 2020, repeatedly telling his base to vote in person at voting stations instead of by mail.
Democrats did the opposite, advising people to opt for mail-in voting in 2020 as a measure against spreading Covid by going to polling stations.
Covid Lockdown ended, and so the option became more of a regular option as opposed to a more necessary preventive one if you are the spread of the disease.
The rest is math.