They've hardly missed their chance. CoC is, so far, fundamentally pointless and only exists to say "LOOK HOW AMAZING THIS CHARACTER IS!!!" Presumably it will have some more interesting uses, but those interesting uses will only be used by like three characters out of the handful that have it.
Luffy's CoC use was a slow burning story over sixty odd chapters where Oda was really starting to introduce the concept of Haki to the readers. We aren't going to get a story like that for Zoro and Sanji. Or Law, or Sabo, who are the other young likely CoC users. These characters will just get one moment in the series that's really meant to be hype as fuck. Or if they're unlucky (Law and Sabo, that is), the Kid treatment of "yeah, he has CoC" without us even seeing it.
For Zoro, his big moment simply hasn't came yet. When Zoro uses CoC, it's going to be a major highlight for him. Everything points to Zoro having a huge role in the end of Wano- that's the moment he'll use CoC, because it'll be the moment that has the most effect on the reader. Storytelling 101. He won't show it against the likes of Monet (though we could have gotten a brief sign there) or Pica, who were well beneath him, and in arcs where Zoro wasn't all that important. He wouldn't show it in Fishman Island, because that was where we got the big Luffy takes out half an army CoC scene and another CoC user would make that moment less special- the same reason why only Luffy used it in the Marineford War, despite there being loads of users and potential users there. Wano, though, is an arc where Zoro's been very important, and it just makes sense for him to show it for the first time here.
Sanji's not as likely to get it as Zoro, but if he does, it'll be after him. Zoro's always ahead of Sanji in their little rivalry, Oda was never going to give Sanji CoC before Zoro shows it.