Part of it is that various states require that all candidates already be registered before now, so it’s Biden or bust in those states - they can’t swap him for a different candidate on the ballots there and they can’t officially transfer any pledged electoral votes for him either if he wins.
There are enough such states to win Trump the election if they go to him essentially by default. And if they all went to Biden despite Biden stepping down then we’d be in a one vote per state election between the top candidates, which leads to a Trump win.
Part of it is that various states require that all candidates already be registered before now, so it’s Biden or bust in those states - they can’t swap him for a different candidate on the ballots there and they can’t officially transfer any pledged electoral votes for him either if he wins.
There are enough such states to win Trump the election if they go to him essentially by default. And if they all went to Biden despite Biden stepping down then we’d be in a one vote per state election between the top candidates, which leads to a Trump win.