there are alot of reasons, first is that Halo 3 is a launch tittle and people who bought the xbox bought it cause it was the only AAA tittle available , Second Xbox owners love shooters most so its no suprise that the commercially succesful Gears was selling.
Ps3 owners however are varied , alot of them like RPG's and some like racers, sports, adventure ...ect the shooter fans are little compared to the PC and xbox 360 but they did grow overtime.
Halo 3 launched 2 years after launch.
Gears 1 was the first AAA title if I remember correctly.
I 100% with your list edit though. Uncharted is not a shooter and neither is MGS4.
It is just like how Tomb Raider wasn't considered a shooter before.
iPhone = Great gaming device. Don't agree? Who cares, because you're wrong.
Final Fantasy VI (iOS), Final Fantasy: Record Keeper (iOS) & Dragon Quest V (iOS)