Wii or DS don't sell more because of their "games library". Casual gamers who buy them don't even know what games are on the console. They know "people buy Wii", mostly because of small motion control party games, and DS has pokemon and casual games. That's all.
They sell more only because they're made for casual gamers and it's fashionable to buy a Nintendo console right now.
The world has a lot more grandmas and kids than there are core gamers, so of course it sells more. Casual things will always sell more than core gamers materials.
It has nothing to do with the library, it's more about "hype" and fashion around Nintendo brand and motion controls.
But for us core gamers, the great games are still on PS3 or on 360, and the handheld to have is the PSP (MGS 5, LBP, Monster Hunter, Valkyria, GTA...). The Wii is still not so appealing, with the exception of Mario and Zelda games.
DS and Wii simply don't have enough technical abilities to bring great games on it anyway.
