Because it was just a games console, when this came out the convenience of a new format came along and it was a popular format that people still use to this day 20 years after.
I am talking about the DVD player in PS2 and XBOX, when dvds and players came in the 96 and later, people kinda saw it as just a meme until the benefits of dvd became more clear like having the same quality as VHS/VCR but a new tape, and not loosing image quality on the films you bought, also at that time one would rent a movies and buy the films one really wanted to re-watch over and over again.
So when PS2 came out, that was a legitimate reason to buy the console, for the stupid dvd player, the same was with xbox but the ps2 had one thing over the xbox, namely you didn't need to having to buy a remote controller to play dvds.
Also, gamecube had another problem that was, no GTA games, or other edgy games that was a must have, they had gems but that was only for the people that knew about these games to begin with.
But any so-called gamers either bought the gamecube or waited until the Wii came out and then bought gamecube games, so to play gamecube games on the wii when the wii market place was too dried up of memorable games, or games of interest.