Clearly, the main reason for this phenomenon was the COVID-19 pandemic—which forced people to seek alternatives to standing in line for hours to cast a ballot. However, it's also true that the Democratic Party was far more effective in promoting non-traditional voting (particularly by mail-in ballot) than was the Republican Party. Also, private individuals—most notably Mark Zuckerberg and Arnold Schwarzenegger provided enormous amounts of money to make the election "covid-safe". Zuckerberg alone spent $400 million in this effort. In addition, several states adapted their voting laws (some controversially) to make non-traditional voting easier.
It obviously worked—particularly on the 'mail-in' side. More than 65 million Americans ended up voting by mail—compared to just over 24 million in 2016. That's a 24% increase in mail voting—which was 41% of the total vote in 2020 and just 17% in 2016. And, the biggest boosts in overall turnout came in those states with the largest increases in non-traditional voting. The question for 2022 and 2024 is: will this non-traditional voting trend continue or will we revert to pre-pandemic patterns? We will know the answer soon—as early as this November.