The reason is simple: Partisans don’t want to release polls that are bad for their side. That means the polls sponsored by a party, candidate or partisan organization tend to be biased in favor of the side releasing the poll.
That’s why it was amazing to find that on average, Trump was doing 5 points worse than he did in 2016 in the states and districts in released Republican and conservative polls.
If Trump actually did 5 points worse than he did in 2016 in the swing states, it would mean he’d lose Arizona, Florida, Michigan, North Carolina, Pennsylvania and Wisconsin. Those states, plus the ones Hillary Clinton won in 2016, would be more than enough for Biden to get over 270 electoral votes.
But remember: These are only the polls conservatives and Republicans were willing to put into the public sphere. There’s good reason to believe it’s worse for Trump in the numbers they’re not releasing.