I guess Kid and Law aren't considered emperors due to their weak crews. Compared to Luffy's crew, outside of Killer in Kid's crew, the rest are fodder, to the point Oda didn't even bother make them relevant in the Wano war
And it's most likely the major reason why Buggy ended up being one. His followers were hyped by Whitebeard preskip
So it was a logical perspective from Oda's part that he'd develop it further to maintain the running gag of Buggy finding an alternate way than individual power to become a top dog
Not surprising at all