They might have sold similar numbers, but the paths they took to getting to that number were different - the Xbox started out as a slow burner, but by the latter stages of the generation it was actually doing pretty well, even if nowhere near as good as the PS2. By contrast, the GameCube sold well for about 18 months and then utterly tanked. Not to mention that the Xbox outsold the GameCube despite selling terribly in most Asian countries.
Plus, while it obviously wasn't anywhere near the top-selling system for that generation, the Xbox did have a pretty big impact on gaming culture, introducing us to the likes of Halo and making people realize just how useful a full online ecosystem was for a time. By contrast, the GameCube, rightly or wrongly, was pretty much seen as just that system that you and you buddies got out whenever you wanted to play Super Smash Bros. Melee.







