Not really. If a hundred people want to play Pokemon, they need a 3DS at £150 and then the game at £30. That's a total of £18,000. If they were to release it as a mobile app, the profit would be considerably less.
Same reason why Apple only make their software for their hardware. Just like Nintendo.
This game really takes me back....
However, you are correct. Nintendo is a hardware company and they will squeeze out more revenue from selling a few 3DS's with some games than even a thousand 0.99 cent apps. Sad we can't have the best of both worlds.