Johnny Awesome
Veteran
Probably because that little black box of joy is still doing very well.. There's still a slew of upcoming AAA titles for the PS2 and the console will continue to get 1st and third party support right into 2007-2008..
So obviously the majority of PS2 owners are probably quite happy to sit tight enjoying there "graphics-isn't-the-be-all-and-end-all-of-gaming" on the console without feeling any significant urge to upgrade their hardware until the PS3 price drops and everybody's favourite PS franchises hit the next-gen box (DMC, GOW, MGS, FF etc..)
Except that everyone's favorite franchises in the US are Halo, GTA, and Madden. MS will have all of that and then some next year at the $299/$199 price point. That's going to be very compelling for consumers - harcore and casual alike. IMO Gears of War is going to be more successful in the US as a brand this generation than MGS will, let alone Halo.