Mail-in ballots helped Biden because so many states, that had never done mass vote by mail before, sent them out to everyone on their voter registration list. They didn’t validate if the addresses were still correct and people didn’t need to request a ballot. So, a bunch of uninformed, passive voters who wouldn’t have made an effort to request a ballot or go to the polls otherwise, voted for Biden because they figured life sucks so vote for a different president.
Biden can thank the pandemic for his election because he wouldn’t have had a chance without it.