So it looks like James Clyburn has endorsed Harris as well as many others, so at this point it’s a virtual lock she will be the nominee. So let’s focus on the question in the OP, what happened?
Obvious answer is that she’s not impressive but most in the 2020 primary weren’t either. There was Bernie and people afraid Bernie would be the nominee and then people angry Elizabeth Warren didn’t drop out earlier and back Bernie as everyone else dropped to back Biden.
The only thing I remember about her campaign was being hyped up on Morning Joe, people being surprised she was floundering, and then the “I was that girl” debate moment paired with t-shirt release.
It doesn’t matter who they put in there. It could have remained with Biden. There is a solid TDS block, and they will punch the chad of whichever name is in the (D) slot.
A lot of people on the (R) side will be holding their noses and ringing up Trump in the same way.
The real race of importance will be Congress. If the presidential winner doesn’t have a favorable congress (both chambers), he or she will be hamstrung.
They all like to pretend that this is some “Dem’s in disarray” moment, but they forget how the nomination coalesced around Biden. There’s no reason to think it won’t be that way again, especially since everybody knows what’s at stake.