Because they made the GameCube more powerfull then the Ps2 and slightly under the Xbox. The problem with the GameCube was not the games though, it was the way it looked (Lunchbox) and awful marketing.
It's sad that such a great system sold so poorly and maybe now we are paying the price for it.
I would much rather pay a extra $200/250 for a machine that is slightly under the next Xbox, then have a machine that's life is cut short a couple of years because games cannot be ported over due to limitations of hardware, 3rd partys not wanting to attempt it's 'lower' hardware, and general crap on the internet about how underpowered it is.
I don't want to have to go to Sony or Microsoft for 3rd party games is what I am saying. We all know Nintendo is about innovation, but who's too say you can have that AND a powerful system.
I want another GameCube, just don't make it look like a lunchbox and it will sell!