There's actually a lot more to Phantom Hourglass's sales than just being on the Nintendo DS (although that is obviously a major factor). Here's a few other points to keep in mind:
- The cel-shaded Wind Waker art style was considered appealing to Japan at the time to begin with
- The touch controls made the game a lot more accessible and got a lot more "non-gamer" people playing
- The Japanese version of the game was designed so tapping a kanji on the screen would display the yomigana reading. This made the game accessible to younger children