Because that's the standard control scheme promoted by tournament organizers that otherwise, don't take an active role in the competitive community, and as such, wouldn't take the time to promote the use of other control schemes. Like at businesses and that kind of thing.
Just take your game cube controller. I don't think they'll mind, and if they do, well that just sucks : )
I came into this thread thinking that it'd be a discussion about how people might turn up to tournaments with their wiimotes + nunchuck and find everyone using cube controllers and getting angry about it or something
