In Dolphin, you have Dolphin and Windows running in the background, capable of operating all input devices and configuring them around. On the Wii, there is no operating system. The game basically runs directly on the hardware itself, like Windows. That means, gamecube support would either need to be coded for each game seperately (since they are all different) or would need a huge complex code to run in all games. As far as I know, MrBean35000vr tried this for Mario Kart Wii (for the USB Gamecube Adapter on WiiU which is not normally supported by Wii games) so it is possible, but just hasn't really been done.