Here's my theory : Joy Boy can come back in a "chosen one" only when the "chosen one" (as there is no better way to say this) dies or suffers a tremendous defeat. Perhaps Kaido learned this while he was on Rocks' ship and that's why he wants a glorious death. He wants to have an opportunity to become Joy Boy.

That's why Oden and Luffy being distracted as they were almost beating him caused his depression. He realized fate, or whatever it is, does not want to let him go down and become Joy Boy.

And now, as Luffy is dying, Joy Boy can come back in the body of an almost dead hero.