The title of this thread, "Why do X360 exclusives sell more than PS3 exclusives?" is kind of inaccurate.
The real question is, why does Halo and Gears of War in particular sell so well? These are the franchises that sell more than the PS3 exclusives. Beyond that, it's pretty even.
It's a pretty simple answer, too, to be honest. Shooters absolutely dominate in North America, which happens to be the one market that the 360 dominates. This is one of the good things about having a centralized market, it allows certain the dominant genre to prosper. Generally speaking, the 360 has a disproportionate percentage of fans who like shooter games. It's the console's bread and butter.
On the other hand, the PS3 is more globally focused, which means that the market is much more diverse. No one genre dominates on the PS3, which means that no one title is likely to come close to Halo numbers. It simply does not have a player base with the same kind of tight focus. Uncharted is a popular game, but it's not a style or genre that every PS3 owner is going to like--I have no interest in 3rd person cover shooters, for instance. The same is true for any other genre.
That obviously doesn't mean, as one poster implied, that games on the 360 always sell better than games on the PS3--just look at fighting games or soccer games--but yeah, shooters will almost always sell better in North America, which means that they will almost always sell better on the 360. The Resistance or Kill Zone franchises would have sold many more units on the 360.
Microsoft also pulled kind of a bait and switch move that I think helped the popularity of their top franchises a lot, even if it wasn't intentional. By releasing several titles on PC and then making their sequels 360 ONLY, you're basically forcing fans of those series into either buying a 360 or missing out entirely. It's kind of like giving someone the first hit of a drug for free. Clever if intentional, fortuitous if not.
