Because it accomplished nothing, it was like: Kid does nothing, Law does nothing, Killer performs one attack, Luffy's attacks do nothing, end then; Zoro scares two Emperors, Zoro unleashes the first attack that forces Kaido to avoid it, Zoro blacks "the most powerful attack in one piece" and continues fighting, Zoro scars Kaido, Zoro overpowers Kaido's thunder with his thunder, Zoro takes thunder bagua with broken bones.
If Kid and Law weren't USELESS in rooftop then I "personally" wouldn't consider it as "fanservice," but since the entire rooftop was Zoro, Zoro, Zoro and again Zoro + still more Zoro, then I don't see any reason for rooftop's existence other than Zoro. It was like Zoro: Kid and Law, can I "chime in" into your fights even though my main opponent in this arc is King? KId and Law: Yes of course, welcome dude. Zoro: can I also steal all your feats and make you completely useless? Kid and Law: yes please!!!