I can't believe how did the PS2 sold so better than GC and XBOX. :-|
It's not all about power of course but higher performance allows for greater game experiences in most genres. The PS2 still outperforms the gamecube in some areas, full dvd drive, 5.1 sound, more main memory, more video memory, more high definition games (gran turismo etc), dvd movie playback. There are quite a few games that are better on ps2 than gamecube just because the gamecube simply hasn't got the storage.
The gamecube was a simple design though with a simple single layer top loading optical drive. It was designed for maximum performance at minimum cost and does that well. Despite its low sales was still profitable for Nintendo which could not be said for the xbox which was often sold at a huge loss and Nvidia screwed them over with the licensing agreement to the point that Microsoft had to abandon it early because Nvidia expected the same royalties on the console even when sold at a fraction of its original price.
To be honest I was always more happy with the games on gamecube and xbox over ps2 mainly I think because the ps2 had such grotty graphics output quality that looked terrible on my AE100 projector at the time.