The PS1/PS2 controller is the 800-pound gorilla of the game world. More people play games with that unit than all the others combined. (Especially when you consider that PS1 controller clones are popular on the PC.) It comes as no surprise that many players who view their Xbox and/or GameCube as secondary to their PS2 prefer to use that controller for everything.
By comparison the market for adapting the Xbox or GameCube controllers to the PS2 is almost non-existent. Connecting the GC controller to the Xbox or vice versa,,, Frankly, you may be the only person on the planet with that desire.