3rd party support and brand familiarity. Xbox does have 3rd party support but due to Sony's past most Western gamers gravitate towards it. And Nintendo, eh they don't think of 3rd parties first but it's working out for them so I guess no harm done at this time.
By the way, those PS1 and PS2 numbers are well off by about 4 million or so for PS2 and 7 Million for PS1. Unless they sold another 4 Million/7Million in the past year I don't know about.