Why buy a GameCube (or Xbox for that matter) when you already have a PS2 that (while not having all of the best games) easily facilitates your entertainment needs? It'll cost you hundreds more to buy a GameCube or Xbox that currently have no more than 50 really buyable games between them. It's not worth the investment unless you're either hardcore or semi-hardcore.
Thanks for confirming what I've been trying to point out all along. Why buy a GameCube if there's not enough software to justify the purchase? On PS2 (and Xbox also), you can slowly get games of all genres. Get a GameCube, and you'll be hard put to find a worthy game in every genre. With Cube loosing support, market apeal is certainly not getting any better.
Wether it's lack of marketing or not, GameCube simply does not have enough market apeal or else it would be selling better.
People bash the GameCube just for the family-friendly approach and the lack of any really controversial games. Word spreads. Soon enough, everybody and their brother is telling people "Hey, I heard GCN sucks.. get a PS2. It has GTA!"
And there you go. This should tell you that the market is changing and Nintendo has not adapted fast enough. Games like GTA sell and those type of games are clearly missing on the cube.