Well, because things are in constant motion and just because a character is in a certain situation at a particular time, it doesn’t mean he won’t get to do anything else. I mean Oda himself said he wants Wano arc to be unpredictable for the readers.
Of course I don’t expect Zoro to just randomly go back, that would be retarded. But a chain of events leading to Zoro being temporarily off the roof or King joining the roof isn’t off the table. And it would lead to an interesting race against time with Zoro trying to finish the fight quickly so he can help against Kaido.