CoC coating is the only thing that gives Zoro the upper hand against Sanji, even by a little bit. Giving Sanji CoC coating will just make him too overpowered even in comparison to Luffy, since it's a great offensive power, and Sanji already has a lot of offensive right now, and room to improve it even further among all the other hints for abilities he might get. I think this is the best reason to not give him CoC, any other makes no sense.
"It isn't on Sanji's character"
Lmao, Sanji is one of the most ambitious characters and has immense willpower. Giving Sanji CoC is literally all just a matter of Oda wanting it or not, but he writes Sanji carefully tho.
Doesn't matter what you think, the writing matters and the manga isn't over yet.
Remember when some people used the "Sanji isn't a Supernova" argue like it made some difference on how he is portrayed and on how strong he is?
I guess that didn't end too well, actually ended in the worst ways possible, most Supernovas got massively clowned from Wano until currently.
If someone wanna fool themselves with color spreads and out of manga infos from decades ago that you think somehow fits an agenda nowadays, be free to do it, this never worked out once anyways lol.
What the manga has been showing us is:
-Oda likes to keep some level of unpredictability and uniqueness for Sanji's character.
-Sanji doesn't fit mere parallels that easily, like Luffy and Zoro do (whatever that means, if it means something, is up to you tho)
Sanji is a pretty good definition of an underdog. He doesn't care for the clout, even if it eventually comes to him naturally (could say the same for Luffy here, in a specific way, he made clear he doesn't wants to be a hero or a ruler)
Oda knows it, and treats it accordingly.