-The PS3 has some critically acclaimed franchises but Sony has yet to create one (aside from Gran Turismo) that has the rabid fan base of a series like Halo. Not for a lack of trying, though. It's just that, for close to a decade, Halo was THE multi-player FPS game of choice and it just happened to be a Microsoft exclusive title.
-Gears of War was quite possibly the first block buster franchise of this generation. When it released, the world had never seen a game so beautiful. As a result, it got attention that it wouldn't have gotten had it released later in the gen. Still one of my absolute favorite franchises but timing has had a lot to do with its popularity.
Aside from those two, I think things balance out for the most part (with a slight advantage in Microsoft's favor). With North America being the biggest market for video games right now, and the Xbox 360 dominating, it's only natural for Xbox 360 games (which includes the exclusives) to sell more.
So Sony "having more exclusives" being the cause it pure bullshit. Microsoft's franchises sell well because they appeal to more audiences. They're more mainstream. Anybody can jump in and blow through Fable 3 with minimal frustration. They're safe. Sony is still trying for that breakout title but they just don't have the formula down, yet. Killzone, Twisted Metal, Resistance, Mag, Warhawk, etc--they just get lost in the shuffle when they compete against third party PS3 titles for the gamer's dollar. Microsoft has these evergreen titles that will always get attention because they seem to be amongst the best of their genres (FPS, TPS, WRPG, RACING).
Given a bit more time, Sony's exclusives will have that same pedigree. Platforming isn't what it used to be (unless it's on a Nintendo platform) so LBP, Ratchet and Clank, Sly, etc. will always underperfom. The only names in FPS right now are Halo, Battlefield, and Call of Duty and they all have loyal followings so MAG, Resistance, and Killzone get lost in the shuffle. Uncharted is a great franchise. Nothing bad to say about that one. Add it all together and the picture becomes clear.
This is the first generation where Sony has actually had to "take charge of the ship" instead of letting third parties take all of the spotlight. They have to create their own system sellers since much of what sold the PS2 is now available on the Xbox brand.
Too long/Didn't read? --They've created lots of new IP's to separate themselves from the competition. It takes time to build new franchises to compete against Microsoft's already established franchises.
