Are you a speedreader or something? Or do you just pay attention to Zoro and literally no other character?
Momo and Yamato are arguably the two most important characters in the alliance other than Luffy. They're the characters who most embody the themes of Wano and they're the two characters who inherited Oden's will, except in completely different and opposite ways. They're the two characters with the deepest connection to Kaido. They're the two characters who are the most likely to join the crew after Wano.
Why else do you think they're the two characters who about to witness Luffy defeat Kaido?
Why else do you think they're the two characters who most reflect both Kaido and Oden?
Zoro doesn't have any real connection to Kaido, despite what some shallow and cherrypicked parallels may suggest. If anyone kills Kaido it will be Momo or Yamato, two characters who have an actual connection to him.
Momo has no chance of joining the crew, if you payed any attention to his character that would be clear. He is not his father, therefore it benefits him nothing to do what Oden wanted to but couldn't, he'll do what Oden didn't want to do which is lead Wano and stay there. Imagine after 20 years of waiting for Momo to return he tells them he'd rather be a pirate, tell me why he's a shoe in to join the crew.
Yamato isn't even close to the most important in the alliance, hell most of the alliance don't even know who she is. If Oda wanted her to kill Kaido he would have tried harder to make her paramount to the arc, by either causing the majority of the conflict or giving her the sole ability to solve an aspect of the conflict. Think of, you know, any other arc dedicated to strawhats or future strawhats such as w7 and ennies lobby, TB, WCI etc.
Yamato doesn't have the ability to kill Kaido, she may have CoC coating but she isn't a swordsman, and we don't know for a fact what her views on killing are. If she can kill Kaido with blunt damage then so could Luffy, and to make Momo a killer while still mentally being an 8 year old is dumb. Neither of them are killing Kaido, they're not strong or important enough or that's not their character, you can pick one.