Two simple things:
a) its got the parental vote (unlike PS3 and 360)
b) its got the zeitgeist
While hardcore gamers will spend more money on games on platforms like 360/PS3 for the big titles like COD4, etc. the simple fact is there are far more families/casual/fun gamers out there in total by far and the console that gets that vote will always take top position - much like PS2 did previously. While PS3 has the brand it just doesn't have the games right now for that demographic - I think Sony really dropped the ball focusing on mature shooters, etc. to compete with MS for that particular demographic instead of making sure they got new versions of R&C, Jak, etc out quickly (and exclusively).
The fact it has the additional boost of catching the zeitgeist just means it selling all the faster.
It was always going to get the loyal Nintendo crowd - but its selling way beyond that demographic right now.
Try to be reasonable... its easier than you think...
