If you upgrade something, it's now capable of doing the things it can do before, and new things. If you remove the upgrades, it can't do the new things. GC on Wii works because the Wii uses the same type of processor (PowerPC), just upgraded. Because they use the exact same method of processing, (reffering to the PowerPC) backwards compatibility was seemingly seamless but in fact the Wii is still different hardware with different component (albeit very similar to the GC) and houses new things needed for Wii gameplay like the many hardware upgrades and IOS files ( different games reference different IOSs, correct?). This would be like having a scooter (GameCube) and a Bicycle (Wii) and expecting the scooter to zip you around like a bike would. Yes they both have two wheels and handle bars, but one is obviously more powerful.
If anything I mentioned in here wasn't correct ( I don't claim to be an expert on this stuff) let me know. Hope this helps.
