A portion of a Joystiq interview with Jun Takeuchi...

Joystiq: The Wii has taken such a large lead in sales, and Resident Evil 4 did so well on the Wii. It's also cheaper to develop games for the Wii. With that in mind, why is this a PS3 and Xbox 360 exclusive title? Was that a creative decision?

JT: Yes. It certainly was a decision from myself and from the development team that we really wanted to push the next part of the series as far as it could be pushed. That included the graphical aspect of the game. So that was the reason that we went with the 360 and the PS3.

Full interview here