I think the big factor underlying this was that Harris as VP nominee was a fait accompli by March 2020. That was Clyburn's price for clearing Joe's path in South Carolina. At the time, she seemed like a natural successor and someone who could run for President in 2024. A lot of people looked at Joe as a one term President. I certainly thought that was a logical path at the time. The problem is that Harris does suffer from the same issues that Hillary has. She's been kind of a dud on the national stage. I don't think that means she would be a bad President, but it certainly makes her a bad campaigner. If Biden looks around in spring of this year and views Harris as a strong successor, it makes a lot more sense for him to say he won't run and endorse her. People could have challenged her, but she would have had the inside track. If he does it now though, it will look like they cleared the field for her because she couldn't win on her own.

FWIW, I actually don't have a problem with the smoke-filled-room approach to picking candidates. I think you'd have better general elections if that was the approach. For example, Youngkin wouldn't have been the GOP nominee in Virginia if not for GOP scheming to make the 2021 nomination process fairly closed. But there's no reverting away from primaries in national elections now.