It's never been about the best candidates. That's a story for school children. In reality it's all about who is connected to who, who makes favors for who etc.
And naturally you need to be rich, which means you are already connected to a web of individuals who you owe favors, or are expected to satisfy in different ways.
In Biden's case a major factor is his catering to the Israel lobby his whole career. They really disliked Obama since he was known to attend pro Palestine events and apparently knew what the deal was. Biden as his VP pick was basically his signal to donors that he would govern like an establishment Democrat, and that he did.
"Best?" Of course not, that's not how the 2 party system works. We don't live in some meritocracy, Capital is what makes the gears turn. The system is designed to change as little as possible.
That's why Unionization and organization at a grassroots level is the only way to move America meaningfully to the left, vote Dem to keep fascists out of office while actually doing the hard work yourself on the ground.
It's a matter of monopoly capitalism and furthering it's dominance in client states. Look at US's involvement in regime change. They sponsor fascist coups who are friendlier to foreign private capitalist interest for the benefit of the US as the global hegemon.