That's pretty understandable too - by the time the Gamecube and the XBox were released, everybody and their dog had a PlayStation 2. Developing for the system made sense, underpowered in comparison or not - it was still going to push those sales like a steamroller.
I forgot to mention that Reggie sticks to specs alone, not the capabilities. The PS2 supported DVD playback, the XBox supported DVD playback, the Gamecube did not support DVD playback. The XBox was Online-Ready out of the box, the PS2 was made Online-Ready via an adapter and later Ethernet was included in the Slim revision to support it out of the box, the Gamecube used a Network Adapter and only ever had four games that even supported Online. One of those consoles sold the least units - hmmm, which one?
Raw numbers alone don't make up for the entirety of the console's real-life performance - it's what the console can actually do that interests the End User. Effectively both the PS2 and the XBox could be used as home entertainment centers as well as consoles, and that was a big deal at the time of their release - the Gamecube? Not so much.
End users should be interested in playing games; that is what the system is for. The Internet can increase the amount of games you can get. But this wasn't as important as it is today. It sounds like the XBox influenced online gaming since the Playstation and the N64 didn't have internet access. Hmmm... we also know that one of these systems had pretty bad third-party support.