Maybe FMV factors into it too.
The majority of companies are using DVD quality videos these days in their crap-tastic games, and they can be easily be thrown onto a PS2/Xbox game.
GameCube however, requires different coding. It would seem everybody is too bloody lazy to learn Factor 5's DivX GameCube compression... So when we do get a port, it has terrible Mega CD type video. Wreckless? Night Fire? Mmmm, chug-zilla FMV.