Most of the admittedly very early analysis I've read is basically it's low trust voters.
Low trust voters are hard to poll because they don't trust pollsters, they don't pick up their phone for strangers.
6 years ago this didn't matter because they were as likely to be Republican as Democratic. But that changed with Trump who appeals to these types of voters.
Then this election democrats suddenly increased their civic engagement which lead to them being much more likely to talk to pollsters. Skewing the results.
I know these are young comments, but I'm fascinated by the downvote patterns I'm seeing here. I didn't think of anything in your comment as remotely controversial; I thought it was the consensus best-guess of why polls are underperforming, as described by very mainstream sources like the NYT.
That would explain change over time, but it has problem with why the problem disappears in between 2016 and 2018 and returns in 2020, so even if it is accurate, it seems to be incomplete.
I think the explanation is basically they corrected for the first problem that happened in 2016 in 2018, but the second problem didn't appear until 2020.
Low trust voters are hard to poll because they don't trust pollsters, they don't pick up their phone for strangers.
6 years ago this didn't matter because they were as likely to be Republican as Democratic. But that changed with Trump who appeals to these types of voters.
Then this election democrats suddenly increased their civic engagement which lead to them being much more likely to talk to pollsters. Skewing the results.