I don't know this for a fact, it's a personal suggestion, the only one way to know for sure is to ask the game devs, but its mostly just basic sales performance, developmental focus and opportunity cost. So its not the fact that I know it, its just my view and how it could easily be aligned with why they most likely didn't make a "Pokémon Z" game because of market strategy reasons. Im not saying its the truth, just a viewpoint.