I may be generalizing, but I think (North) Americans are more into "macho" games like FPSes and sports games, genres that don't seem prevalent on the GameCube. These games are also more on the realistic side. Playing realistic games where you shoot up or tackle people are the kinds of games the Western audience seems to want. Hence, people will give the N64 a boost when they can play games like GoldenEye or Perfect Dark, but will salivate over Halo for the Xbox.
I don't know what kinds of insecurities people have, but apprently a lot of people here in the West won't play cutesy games. As gaming becomes more mainstream and more social, I think people become more tied down to traditional perceptions...they're not "mature" if they're levelling up Pikachu. And I must admit a bit of that myself... I may be a big Pokémon fan, and have in the past been vocal about how it's a game experience worth having, and that you shouldn't take the cartoon and merchandise as an indicator as to what the game is like, but seeing as how I'm 25, most of my peers never would never consider it. So even if I felt I should openly defend my quest to catch them all, it'll fall on deaf ears and make me look like a lunatic. Those that I know have (in the past at least) won't talk about it openly. If you HAVE to talk about video games in a social setting, it's easier to say you blew up some aliens' heads than that you managed to catch Mewtwo without using the Master Ball.
And those games that Nintendo has that aren't cutesy and should appeal to the insecure crowd? Well, Nintendo doesn't market them enough.
Anyway, about the Revolution - the FPSes that will be capable on this should help make up in that regard. Unfortunately, insecure people might now feel like they're holding a toy gun. Light-gun games aren't as popular as FPSes, it seems, probably because of that reason. There's no pleasing some people.