I think at this point, because of the market saturation of mobile devices, it doesn't quite make sense to create another gameboy or psp-like mobile gaming device. Everyone has a phone, and an app like PokemonGo is in part so successful because there isn't the need to purchase additional hardware.
With an app updates can be pushed to improve the game. I for one look forward to new updates on the app and improvements to the current performance.
Hardware makes fast updates difficult and I can see why they chose to stick to a mobile app. Gaming hardware has been a fairly tough sell in recent times.
A push for Nintendo games on mobile platforms would be great and possibly would revive interest in their content.