Exclusives, man. Can't just look at one series. Look at everything UNIQUE to that console.
Let's say you're a shooter person. You don't pick between a 360 or a PS3 because "omg I want COD." NO. That's stupid, since BOTH have it. You'd need to look at what each offers on it's own, like Halo vs Killzone and Resistance. (I'm sorry I don't have more examples for each side, I'm not a shooter person.)
This is why it kind of irks me when people say they love their console because of games X, Y, and Z, and they're ALL multiplatform. I mention it and ask about exclusives and they spaz the hell out on me, like what?
Point being, this is why Nintendo is still strong. They have good first party support.