One hypothesis

Blame it on complicated friend codes or a lack of vauntable trophies and accomplishments, but according to a survey conducted by The Diffusion Group, only 54% of U.S. Wii consoles are connected to the Internet. This makes it the least connected current generation system, trailing behind Playstation 3 (78%) and Xbox 360 (73%). The same report states that one third of U.S. households now own an internet-connected console.

Perhaps if Nintendo of America tried out some inventive promotional campaigns, U.S. Wii Remote-wielders could remedy this problem, but as of now it seems these American gamers are happy being virtual homebodies.