The gamecube didn't do very well in the US compared to the Xbox.
http://vgchartz.com/worldcons.php
44.86 million PS2s
16.34 million Xboxes
12.44 million Game Cubes
It's hardly fair to say "The gamecube didn't do very well in the US compared to the Xbox". 4 million is not a huge difference compared to 28.5 million difference between the Xbox and the PS2.