Harris was the vice president, and was therefore the closest thing to a small-d democratic choice amongst the available options. Otherwise... Why Harris and not Newsom?
The better choice would have been Biden stepping out earlier and having a real primary, of course.
That’s why Biden should have picked a vice presidential candidate who could run a good campaign in 2024. It’s not like Biden’s age was an unknown factor. Biden himself floated the idea of only running for one term back in 2020.