They don't trust voters not to choose a succdem and offend pwecious widdle capital, and they're worried they couldn't fix a second election after 2020 almost went to Bernie despite their best efforts. So now they're just propping up the old man strategically until it's too late and they can put whatever blob priest into office they like without the unwashed masses messing it up with their "democratic will" or whatever. Technocrats must sit the eternal throne whether the people want it or not.
If the Democrats were an actual party I could see it.
They aren't. They're a loose collection of cults of personality built around a few key figures, there aren't really "Party" elites because their connection to the Party is merely a rung they climb to their own personal branding. Obama was able to help Biden win the primary because his personal brand is so strong among Democrats, not because he gives a shit about running the Party or advancing the Party's agenda. Even stopping Bernie was more of a way to protect his legacy as the Hope and Change President.
If Obama had not told Biden not to run in 2016, Biden would have been elected and would not be running in 2024. The 2016 and 2024 elections are Obama's fault
Synthesis: some of those clusters thought their chances were better if they skipped the primaries and waited to insert their person whenever Biden keels over.
(But I think they were largely caught off guard with how early he sunk.)