Pokémon Sun and Moon - Pokémon Ultra Sun & Pokémon Ultra Moon - Nintendo 3DS - Nintendo Direct 9.13.2017

Game Freak has this odd stipulation of waiting till the next system has a large enough install base before the main series of games arrive on it. Which, considering how said games absolutely skyrocket sales themselves, is rather ironic.

On a different note, it's been mentioned by many that the reason we still don't know a whole lot about USUM is because there were a LOT of complaints about how there was nothing left to discover in ORAS when those games came out since they revealed every last detail (even the Delta episode).