Mostly what Rol said. GameCube was Nintendo trying to imitate, which Nintendo doesn't do that well: the controller looked toyish compared to the competition, punier discs (though as Rol said, GameCube disc space really didn't do much damage to the console, except that they and third parties couldn't apply FMV as liberally, but we know how much FMV is worth to sales), lack of online *was* what let the Xbox move ahead, as the people who flocked to the N64 for their multiplayer action largely drifted off to Halo
Playing Mario Sunshine again, you do recognize the mediocrity compared to the other 3D Marios. Sunshine's world was more "real" with citizens and fairly believable environments, even taking away FLUDD, it didn't feel like 64 or Sunshine did.
Metroid, of course, was sublime, but that series has always had limited mainstream appeal. Ultimately the weakness of Nintendo's own first party titles coupled with their console being a features-inferior copy of the competition meant you really had no reason to buy it. Thus the GameCube defined the Nintendo Core, because it reduced Nintendo's fanbase to those who liked Nintendo's development style and IPs as things good in and of themselves, and people to whom things like FPSes or Grand Theft Auto had limited appeal (also inadvertantly creating a core Nintendo fanbase inherently wary of third party games)
Monster Hunter: pissing me off since 2010.
