I dont know the exact figures but it's got to be down to the fact that the DS already has such a huge user base as they've done the same with Black & White 2 as well. The 3DS has sold quite a few but nothing compared to the DS yet and people with a 3DS can play (and buy) DS games as well.
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XBOX360 Gamertag :Kitanode
Currently Playing DS - Zelda: Skyward sword, Halo: Reach, Soma Bringer, DS2x86, Terraria, TW: Shogun2