Probably becasue there are no games on the PS3 that are equivalent to Halo and GoW1&2. Those 3 games almost increase the 360's attach rate by 1.
Oh, and Sony's marketing is crap, I can't believe that they had more adverts in the UK last year than this year...
PSN ID: T_Gears
End of 2009 ltd sales:
Wii = 67-68m
X360 = 38-39m
PS3 = 34-35m
Prediction: The PS3 will surpass the 360 on weekly sales after it drops to $299 on all regular weeks (no big releases).