There is most definitely a relationship between candidate skills, job requirements, and interview result/job performance.

This is actually not at all an obvious fact. It's continuously offered as a ground truth, but many people dispute it and a lot of successful organizations do not recruit weighing these factors as heavily as you're suggesting.

The answer is simple: they likely weren't good enough. But no one likes hearing that.

Possibly, or possibly they were plenty good but so obnoxious or outrageous that they wouldn't be welcome. I've certainly done that more than once in my time building tech organizations. I still remember the guy who effusively praised the beauty of all the women he saw and congratulated me on "the haul". Even touched a woman's hair to compliment it. Too bad he was such a sleazebag, he seemed smart. But even from a cold economic standpoint the cost to the company for an inevitable sexual harassment lawsuit would always eclipse any value he could provide.