It's purely a price issue. Smash Bros is at its best as a multiplayer game and I enjoy offline gaming. I've sold my GC which is why I don't have GC controllers any more. I surely won't buy classic/GC controllers just for playing Brawl's multiplayer offline. Excluding Wiimote/nunchuk controls would definately hurt Brawl's sales too.
Alright, then I can understand it. This really is an issue, i haven't really thought about that... Best thing would be that they sell it with an included Classic Controller.