Wasn't that pretty obvious? Biden's faculties already made zombies look intelligent in 2020 and he ran on not running a second term. His victory also showed that a corpse could win with the recognition of being VP to a relatively liked president. (that wasn't what happened but I'm sure that was the takeaway for a lot of liberals) Finally, anyone who looked at the margins biden's victory (44k votes on 3 states) could see that without the advantage of Trump fucking up COVID response and not being able to campaign due to pandemic rules, he'd have it in the bag unless democrats got someone who could fire people up, which Holocaust Harris never was.
The only thing that I didn't see coming was Biden running, his debate performance reminding the billionaire backers he died in 2019 and being forcefully replaced by Harris. That was wild as hell, people suddenly realised they were being governed by one of the undead and just anointed his VP.