There are a lot of developers and publishers that ignore the 3DS despite its popularity. Western support is for want of a better word virtually non-existent. For a game like Call Of Dury its probably more to do with the fact that the hardware wouldn't allow a quality game. Just look at the Call Of Duty game on the Vita. Its hardly a stellar game and the Vita is far more powerful and far better suited to an FPS game.
From all the criticism i read about Vita`s game, the problema wasn`t that the game was bad visually, but bad overall.
If 3DS can put out games like RE Revelations, i`m sure it doesn`t lack power to deliver a good CoD experience.
I think that Activision could do better on 3DS with CoD, than did with DS.