there was many reasons.
GC was the last system realsed of the ant.. generation
The others systems have DVD
The PS2 and the Xbox were too easy to piracy.
There was not a good marketing or advertisement. PS2 made a strong campaign of his sytem.
Not support from 3rd parties.
Ninty Games are so good but that was the age for teens, and teens prefer, bullets and blood than colours and happy music.