Because there is no role or responsibility of being the VP. Thats what yall keep missing.
The president had died in office what, a handful of times in the past 250 years?
Nobody realistically expects the VP to take over from the president. Not really. The last time it happened was the sixties.
Even with Biden's advanced age, I think they ran his medicals and see that he's projected to live until he's in his 90s like all recent presidents. I mean dude gets the best medical care in the country.
So with that being said, the VP is nothing but a ceremonial figurehead. That is the role of the VP except in extreme situations like Cheney. But historically the VP serves at the pleasure of the president. If the president wants them to do something, then that's what they do. If the president wants them to do nothing, then that's what they do.
So there are no "basics" that you have to handle during a primary to be able to qualify for VP. Heck Joe Biden did terrible in the primaries in 2008. He did even worse than Kamala as far as being a favorite. At least Kamala had a spark when she started. Biden was a nonfactor from the beginning and Obama still chose him.
Why? Because the VP is just a figurehead who serves at the pleasure of the president.
What went wrong with Kamala is that when Biden tapped her to go to the plate for what should have been relatively simple, she bungled it.
I'm sure that made him and everybody in the white house go

Then they pulled her back. They're actually doing her a favor in my opinion by not letting her embarrass herself further.
But that had nothing to do with her primary showing.