There are several reasons the afforementioned games sold that well on Gamecube:
1. Lack of other quality games
2. Lack of adequate quantity of games
3. SSB was a Gamecube launch game
4. Gamecube install base was more "hardcore" than the Wii
5. Gamecube's attach rate is way better than the Wii
Also :
1. Final Fantasies sold better on PS1 than the much bigger userbase PS2
2. Zelda Ocarina of Time sold way better on N64 than Twilight princess on Wii & Gamecube
Install base isn't everything although I believe that both games will eventually sell better than their Gamecube counterparts.
My themeforest portfolio: