I also have a theory as to why 3DS is coming first that has absolutely nothing to do with current sales.
I believe there could be full automatic connectivity between 3DS and Wii2. And by full I mean that I could play MH on 3DS, and if I have the game on the Wii2 as well, they will synchronize anytime they are near, I will be able to continue the game where I left on the Wii2 (and vice versa). Imo such a feature would be really something for the modern gamer, and would be a great incentive for many people to buy both the 3DS and Wii2.
So, Nintendo's plan would be - get a great lineup of games and developers for the 3DS, get lots of people to buy a 3DS, and then close the trap by coming out with a Wii 2 with full connectivity as described, so all the developers on board for the 3DS would have incentive to develop those same games for the Wii2 as well (maybe some already are?...), and gamers would have even more incentive to buy the Wii2 due to its having great 3rd party support and full connectivity with their 3DS...
You read it first here (ok, I did mention this theory of mine in some other thread several months ago)
Currently Playing: Shin Megami Tensei: Devil Survivor Overclocked, Professor Layton and the Curious Village
Anticipating: Xenoblade, The Last Story, Mario Kart 7, Rayman Origins, Zelda SS, Crush3D, Tales of the Abyss 3DS, MGS:Snake Eater 3DS, RE:Revelations, Time Travellers, Professor Layton vs. Ace Attorney, Luigi's Mansion 2, MH TriG, DQ Monsters, Heroes of Ruin