I think the issue with eye or face tracking is that it is not a casual thing for a computer to do. It takes processor time to accomplish it.  Now, the use of a dual core processor prevents this work from interrupting your game or other application, but you still get more of a tracking delay.  There is also a delay as the USB camera sends its image over the USB cable.

The Wii Remote has its own internal camera, and sends very minimal data over the USB.  I don't know the processor speed, or it tracking speed, but since it is getting a much cleaner image of the IR LED's than what the head tracking program gets, the work needed to extract the location of the LED's is greatly reduced.

Joe Dunfee