Why do you think Rotation Battles sorta just stopped being a thing?
Well… well, I want to just answer “because they were kinda dumb and gimmicky and probably not worth the effort,” but upon sober reflection that might be slightly unfair. I don’t think alternate battle formats with different fundamental rules are in principle a bad idea; they mix up what can be a somewhat repetitive core gameplay experience, and depending on exactly what rules you change, they can be ways for otherwise useless Pokémon to get some time in the spotlight (there are quite a few that have never been good in singles, but shine in doubles because of their support skills). I don’t know much about how rotation battles tend to play, though, because… well, because hardly anyone ever played them. Even in generation V I don’t think there were ever many big tournaments that used rotation battles as the format, or a large competitive community. And… well… even the games themselves sort of treat them as a gimmick. There are so few rotation battles in the single player story that you never really get a feel for how they’re different from single or double battles, so it doesn’t feel important to learn how they work and there isn’t any case made for why anyone would want to play them. They’re just… kind of superfluous, and to make them not superfluous they needed to have more support from the very beginning, not just be kept around for the sake of completeness.
I have also seen a suggestion that axing triple and rotation battles for Sun and Moon might have something to do with the graphical capabilities of the 3DS. Triples and rotations in X and Y are… well, they have performance issues, put it that way, and Sun and Moon noticeably struggle to run at normal speed even with four Pokémon on screen in a double battle. I don’t know that this was a factor in discontinuing them, but it kinda makes sense to me? The Switch is more powerful and Sword and Shield don’t seem to have these problems, so mayyyyyyybe there’s an argument there for the return of triples and rotations in a future game? Not sure.