It was because of it's motion controls but he has said they have done as much as they can with the series on the Wii and want to push the series further. That's why he stated they are looking into other platforms for the series and since he is really excited about Microsoft's Natal I wouldn't be surprised if he either ends up making a NMH with Natal in mind or will bring the series over perhaps using the motion controllers made for Xbox 360 that comes out this Fall (with a little game called Squeeballs). Then make a totally brand new ip using Natal.
How our favorite systems are just like humans and sometimes have issues finding their special someone...
Xbox 360 wants to Kinect & PS3 wants to Move! Why are both systems having such relationship problems? The reason is they both become so infactuated with desire while watching the Wii as it waggles on by. They simply want what they can't have.
Official member of the Xbox 360 Squad
