I think the main thing is that the 3rd party developers are more interested in making stuff for the 360 and the PS3, even if the fanbase is bigger for the Wii (although I doubt that the Wii's fanbase will ever beat the 360 + PS3's). If a developer wants to make a cross-platform game, they are going to have to make something completely different for the Wii, compared to the 360/PS3, which means that they are going to be more likely to opt for a cross-platform PS3/360 game.
As time goes on, the gap between the 360/PS3 and Wii will widen, and many people will tire of the repetitive Wii games, or something along those lines. As the 3rd party support for the Wii will be weaker as they cannot get anything more out of the Wii, and the games are going no-where, their main support will be for the PS3 and also the 360 (here comes the arguement concerning power). Therefore, the battle between the 360 and PS3 comes down to the features of each, and primarily the first party games (of which Sony has the best in my humble opinion)
This is why people think that in the long run, the real battle is between the 360 and the PS3. (P.S. Sorry for probably confusing you, I got a little lost)
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