The reason why Zoro wasn't in the Whole Cake Island is that Zosan would finally become canon and not even Oda-sensei's rule " no ships between straw hats" could prevent it from happening.
Just picture this: Zoro searching for Sanji, describing him as the annoying curly brows that occupies too much of his mind, the stupid ero cook he has to save because he can't live with out. So everyone on the island gets the memo that Zoro is madly in love with Sanji (true) and that they are boyfriends (not true yet). Big mom's minions say to Sanji " you know that your boyfriend is gonna crush the wedding, right?'' to which Sanji goes "???" because unfortunately he has no boyfriend.
Then during the wedding, the straw hats crush it and Sanji realises that the boyfriend everyone was talking about is his secret crush , Zoro. At this point, Sanji.exe has stopped working , Zoro is worried that the enemy did something to Sanji to make him look stupid (he doesn't look stupid mean Marimo, that's his happy in love face!!) and he slips his tongue and says "what did you do to MY sanji?". Now Sanji is drowning everyone with his nosebleed, Chopper freaks out because Sanji could pass out with so much blood loss but Sanji doesn't mind because he's the happiest he has even been. Zoro still doesn't know why Sanji has such a stupid look on his face but since he got his cook back he doesn't really care.











