Accumulated damage, Kaido has been damaged by the scabbards, roofttop folks, Yamato and Luffy.
Its implied that Kaido is abit weaker than he was before, and Luffy is having spewing blood for each direct hits, the damage over time will add up.
Theres no reason for nor is it possible for him to recover from such injuries quickly.
Yall just ignore the story to blindly ignore the story and its credibility to fanboy over the thought of ZKK, fairy tail level nonsense, then again random Mink medicine was also fairy tail level.
Even as a Luffy fan I don't think Luffy will KO Kaido but Zoro will not return