I bet I know why they did that, I think they know they are taking a risk with Wii (which they mentioned a lot) so for the people who aren't getting Wii, they made it for Gamecube. Did they explain the controls for the Gamecube version of TP, I know they explained the Wii controls. I'm not sure making TP for both systems is a good idea...
They did it because they already promised a long time ago that Twilight Princess was going to be for the Gamecube. Only since E3 did we find out it was going to be for the Wii.