I think it's partly due to the way Sony thinks about the product. If they had intended on revolutionizing the way we play games, they would have marketed it differently, and made sure that there were better games for it. This way it could have become an extreme hit, providing that the general image of the PS2 would not have gotten in the way.
It's the company's attitude towards these things that is the first and foremost reason that Nintendo is selling millions and Sony is not (not that they weren't/aren't, I just don't know how to say it differently).
EDIT: Basically, what I'm saying is that if Nintendo had made the wiimote a peripheral and tossed it in with a couple of lousy games, they would most definatly be last this generation.
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