Microsoft has more money to market their games. I mean I saw more adds for Crackdown 2 then I did for God of War 3.
Sony and Microsoft are at battle at making "the" home theatre station. They both want their console to be the centerpeice of your livingroom. Except Sony went all out, making a PS3 Blu-Ray before Blu-Ray was cheap and practical to produce. They put too much in, and it's hurting them. Only recently have they seen a profit. Microsoft simply wanted to control games, and influence movies. I think a 360 successor will try to be "the" home theatre system, but with the 360 Microsoft simply wanted to make a name for themselves.
To put it simply, thats what they are doing, convincing people that they are "the" gaming platform for gamers. Yes Wii sells better, but 360 is the gamers system.
Microsoft still puts out as many great products as Sony does, this year I see 7 big exclusives on both consoles. For 360 there are Halo, Fable, Mass Effect, Alan Wake, Splinter Cell: Conviction, Crackdown 2, and Metro 2033 (I know there are more). For PS3 there are GT5, LBP2, God of War 3, Heavy Rain, MAG, ModNation Racers, White Knight Chronicles (Again I know there are more).
However Microsoft has Call of Duty limited bundles, early map packs for 3 more years, and they do stuff like this with all big titles. They have enough money, 360 generates cash, and Microsoft spends it making a name for themselves.
So that is why 360 titles are always soo successful saleswise. Sony on the other hand, I blaime a lack of a solid fanbase. Hear me out, an FPS on the 360 will sell extremly well because the 360 is full of FPS nuts, and has established itself (through a few great early titles, and marketing) as the Shooter console. However Mass Effect, Lost Odyssey, and Tales of Vesperia didn't sell that well. Thats because the 360 lacks a significant RPG fanbase. Don't get me wrong, I love my RPG's, especially the 360 installments, however I can see why a lot of die hard RPG nuts don't relate Microsoft to RPG console.
So for the PS3 FPS don't sell well. No matter how good Uncharted, Killzone, and Resistance are, there is a lack of a PS3 favoring FPS fanbase. That shows in it's regional sales (Most PS3 fans are from EMEAA, a place where ALL FPS just don't do that well).
However Racing games sell well on the PS3, so do RPG's. Take a look at GT5 Prologue, and Final Fantasy XIII.
In conclusion, 360 games are successful due to marketing and a strong FPS fanbase, PS3 games generally aren't as successful because established fanbases are for smaller genres like racing and RPG. Many of PS3's past exclusive titles have gone multiplatform as well (See P.S. below)
P.S. for Sony established mega franchises, Final Fantasy was exclusive to Sony during the PS1 and PS2 era, MGS still kind of is, Gran Turismo is also a big Sony branded game, and until GTA IV came out, GTA was largly associated with Sony. God of War is pretty epic still, and so are Kingdom Hearts and Dragon Quest. Resident Evil was mega on the PS1.