Sales and expectations are weird things, especially when it comes to Pokemon.
1 - Whether releasing on new hardware as a launch title or releasing on a system saturated in the market at the end of its lifespan, the outcome means different things.
a. For instance, releasing on NX would get Pokemon fans to buy the console... but perhaps others don't give a damn. Thus, a company is worried about not selling as much.
b. However, as we saw with Black/White, releasing on an existing system doesn't mean all owners automatically buy. 150 millions DS's out there, asking for each person to purchase a game just doesn't happen.
2 - So what this comes down to is the audience of Pokemon, which -- while huge -- shifts from system to system and generation to generation. Run the numbers to find how much of those are new fans and how many are continuing players. That's your sales formula.
3 - The ball is in Game Freak's court, which is key. It's not in Nintendo's. I bet GF, like a third-party might, has to look at the next system (NX) from an outside perspective. Who knows if they had been specifically working on the console from its inception behind closed door, or if they were left to fend for themselves on 3DS, n3DS, or Wii U.
If you're a fan, the perfect world is some combination of: NX (releasing this year) + Sun/Moon (releasing this year) + Fully-realized graphics and functionality. But if you're a company, learning a new platform takes a ton of resources. And for a rather smart but careful-with-their-cash group like Game Freak, they might not see the benefit.