As said before, Game Freak decided to make all of the models in the game to have higher polygon counts in this one. Sadly, this drastically decreases the framerate because the game has to process so much more. They didn't include 3D in Sun and Moon because even the N3DS would have exploded from the amount of shit they have to render at a time, from Pokemon to Trainers to Battle Effects to Totem/Ultra Beast auras (at very situational times) and the background, all of which are much more dynamic than they were in XY and ORAS.Again, give us the option. If it truly does suck, then I wouldn't use it. I think they were just lazy, no reason we couldn't have the option like in gen 6. And I'm certain that the N3DS could handle 3D in the overworld, its far less complex whats happening out there than in the current battles. Again, I just think they were lazy, but hey I get it, they were focusing on a ton of other new features and things, working in 3D was probably on the last of their To-Do list.
tl;dr too many polygons N3DS would have dies if 3D was on.