For one, they have fixed issues that indignated the GC.
The price is (or was, when released) way below the other two consoles.
It's Nintendo.
It released with a GREAT bundled game, Wii Sports.
The reason, however, that made the Wii such a success is the fact that it included motion sensing. What you hear from adult people and the more elderly in regards of the Wii is "Yeah it's the game where you can move and stuff". They may not have any idea of what the Wii does, who makes the Wii or what games are on the Wii. The simple fact that the console detects motion makes it that much more simpler than any other console to use for those who have never played. An outstanding example has to be my dad. He never liked my N64, GC, Xboc or anything before them. He would, at most, sit behind me and see me play. Now, he comes in my room or the living room and says "let me try it" and plays, even if it's for just 2 minutes, those 120 seconds are more than what any other console before the Wii have done, and that I consider to be a great, GREAT step.
That is the reason the Wii sells so much, along with the fact it has great games, attractive to kids, teens, young adults and adults. To say the Wii is the ONLY console to attracts kids 12 and under may hold true to a certain degree, but this statement doesn't mean it wont be, or is, appealing to the other, more reserved, demographic: adults.
