Oda revealed why Luffy doesn’t use nicknames for his Nakamas

D: Heso! A question! Luffy gives nicknames to all sorts of people, however, why doesn’t he do so with his crewmates? P.N. Takataka

Oda: Ah, I see! Well…Luffy’s a type of person who, basically, doesn’t really care about remembering someone’s name, so he’ll give people a nickname based on his impression of them whether they like it or not.

But with that said, when someone becomes his crewmate, he finally remembers their name. He knows their names, so he calls them by their names. That’s really all there is to it.