Democrats didn't elect Biden, Republicans did.
There was a very well organized and funded group of conservatives that were appalled by 45's behavior and performance while in office. These were establishment conservatives, think tanks, republican campaign strategists, and the like (groups like Lincoln Project and Republican Voters Against Trump). They pooled their money and talents into a very few critical counties in GA, PA, and AZ.
In each of their targeted counties, they peeled off about 6-8 percent of R's and switched them to vote Biden for this one election. They even coordinated with Stacy Abrams down in GA to keep out of each other's lanes and maximize the get out the vote effort for disaffected Republicans. I have a suspicion they dirtied up Brad Parscale so that in leaving the campaign at a crucial time, they wouldn't have the analytic skill set to see the shift in the numbers game.
It was brilliant.