This one of the things I hate about 3rd party developers on Gamecube.
They hype up what is potentially a good game, then just tear it down by cancelling the Gamecube version. It's happened before, it's happened now, it's going to happen again. The public view Gamecube as the console with the least variety. Developers don't want to develop for Gamecube because of this reason, and that their game might not do well on the platform. It's a paradox, and this same thing can be said about Nintendo being 'kiddy' (just replace 'least variety' with 'kiddy').
Also, developers potentially making a game exclusive to Cube, just go on and make for the other two. Like Batman: Dark Tomorrow and The Hobbit.
Thing is, I only depend on NIntendo for my games. If there is a 3rd party game that interests me, then I might consider buying it.