The reason that PS3 exclusive game sales are not as high as the 360 has many reasons in my opinion. but i think this is the main reaason...
PS3 owners tend to play a variety of games. what i mean is that you have many people who bought the system ONLY for RPGS or only for platformers, shooters, action etc. Where a very very large portion of the 360 owners bought it entirely for shooters (not all of them of course) Therefore just because Uncharted 2 is a great game doesnt mean that all the PS3 fans are interested....