GT5 did not outsell Gears in America and Uncharted in America is NOWHERE near Gears. Don't know what crack you're smoking to even suggest such a thing.
I don't care about discussing Europe, as I know Sony does do better there. But there's something with their games that are just not appealing to America.
This is gonna be a somewhat controversial opinion... buuuuut... ALTERNATIVE THEORY DEVIL'S ADVOCATE TIME ASSAULT GO
What if it's got to do with Europe's pretty big bias against the Xbox? Several of my European friends have said that the states of things in Europe are generally pretty pro-Playstation or PC as opposed to Xbox. Obviously, anecdotal evidence isn't as valuable as statistical evidence, but I think it's a point of view worth considering.
If this is the case, and Europe is just biased against Microsoft (not counting Great Britain, which seems pretty positive towards the Xbox brand) and/or biased towards the Playstation... well, you'd see PS exclusives do better in Europe as opposed to Xbox exclusives.
In America, where there's no particular bias against Microsoft, perhaps then it comes down to the quality of the games, where Microsoft kinda easily comes out on top.
Forza is just a better driving game than GT5 (GT5 is the better sim, mind you, but sims and consoles aren't exactly best buds), Uncharted's gameplay is really weak, with poorly designed levels and boring weapons, as opposed to Gears' genre-defining gameplay (Gears 1, which I'm playing now, is horrible, though), and Halo pretty easily holds off Resistance and Killzone when it comes to first person shooter design.
Honestly, if games like Killzone, Resistance, Uncharted, and Gran Turismo weren't exclusives, I don't think they'd have the reception they've got as exclusives. Contrast this with Forza, which got me into driving games, Halo, which defined shooter design on consoles (and has some of the best AI in shooters, period), and Gears of War, which spawned countless imitators.
I don't think Sony's exclusives have the strength to contend with Microsoft's, and the reason they don't do well in America is because America doesn't have a latent anti-Microsoft or Pro-Sony bias.
Again, this is me being the devil's advocate and all that. Could be totally off-base.