Th real question is why it cant be used for gamecube games.
The real answer to this question? They want you to have to buy a Gamecube controller to play Gamecube games. It's that simple.
Also, for those who are wondering, the Classic controller does work a lot better for many of the older games than the Gamecube controller. If you remember, the Gamecube controller has a pretty lame d-pad, and those older games often require a good one.