I don't see why it would be gamecube controller only, not everyone owned a gamecube last gen (like me).
Now i know they have more then one control scheme but in my eyes they should use classic controllers and gamecube controllers, this is because:
a) they have stopped producing them - hard to find one if any in a store
b) classic controllers are cheaper then gamecube controllers - yes i know they need the wiimote to attatch to but who doesnt already have atleast 2+ wiimotes
anyways, just my 2 cents!