The reason I believe Sony's exclusives can't sell as well as they should this gen.

[freebs2]

I can think of 3 main reasons

1.)The difference between Sony and Nintendo IPs or Halo is their approach to game development.
When Nintendo has created Mario, Zelda, Mario Kart or Smash Bros they were proposing an original game concept (or they have re-defined a particular genre) in search for an audience. In other words with Mario/Kart/Smash etc. they have "created" (or "found") a new audience for the specific kind of game.
In they same way, when Microsoft proposed Halo they have created a new audience for online FPS gaming on consoles. And, in the same way, when Sony proposed Gran Turismo they created a new audience for driving simulators. In fact both games have a huge success.
On the other hand, when Sony releases games like Mod Nation, Killzone, Resistence, All-star battle royal they aren't going for a new audience, but they're trying to steal (or to conquer) a share of an audience fed by some other game.

2.)Another very relevant reason is the general target audience, Nintendo most succesfull IPs are made for everyone (child/young/adult male/female) while most of Sony IPs are too gore to be considered appropriate for childrens and, at the same time, to juvenile to be considered appealing to the adult (I think of God Of War, Twisted Metal and Infamous).
You can take Gran Turismo again as the opposite example, GT can be played by everyone kids love cars, youngs love cars, and also adults love cars. And, as a matter of fact, GT is Sony's most succesfull IP.

3.)At last, Sony hasn't learned very well yet that multiplayer sells games and sells consoles.
WiiSports, Mario Kart, Smash Bros have sold tons due to local multiplayer. Halo and Call Of Duty have sold tons because of online multiplayer. That said, where is Sony's multiplayer killer app?