The gamecube was a miserable failure. It was only close to the xbox in numbers because the Japanese didn't buy ANY xboxes. If it wasn't for the DS and GBA doing so well, Nintendo would have definatly went the way of Sega. It was largely the fault of those stupid mini-DVDs. Developers couldn't make the games equal on the GC as to the xbox and PS2, and for homebrew you ended up with a franken-machine in order to fit full size DVDs. Xbox didn't have the exclusives, but with a few exceptions, if you got a multi-system game, the xbox version was the best.
That isn't to say that there weren't a bunch of good first party games for it, I would even go so far as to say it has better exclusives than xbox did. But none of these exclusives were as good as what the N64 had. Comparing Wind Waker to OOT and Mario Sunshine to Mario 64 is blashemy. Mario Kart and Mario Party were good as always, but weren't significantly better than on N64. Out of all of the consoles I owned, it was definatly the least played of all time. As much as the GC was a party system, I found myself (and friends) playing the N64 versions of the above games on the xbox.
While the hardware is the same, the wii is different. It the machine for the gamers who don't have time for in depth games anymore. I sat down to play lost planet on the 360 a few days ago, I had an hour, the intro and opening sequence was >30 mins. I love games getting all cinematic, but an amazingly fun game that you can just sit down and play for 30 mins, is what people need, and that is what the wii delivers.