People will buy platforms for the games you just need a bandwagon for them to hop on. Just because people play Angry Birds on iPhones doesn't mean some equal craze isn't going to drive them elsewhere, Nintendo has just not created such a title.
Nintendogs + Cats was probably held back because the original audience wasn't the early adopting type and to make a causal friendly ecosystem you need a low point of entry, which $250 wasn't low enough for them. Once initial hype dies, people don't come back for second looks. If they were to release Nintenponies this November they'd probably do 3x Nintendogs + Cats easy.