I think it's why XBox has done so well in North America and not as well in the rest of the world. The US in particular seems to have a lot of one-dimensional gamers. I live in Ohio, and when I talk to some of my friends about gaming, a lot will say "Yeah, I'm a gamer too". When I ask them what they play, a shocking number say the exact same response "Call of Duty and Madden." SMH...
To link this back to the point in the OP, since it feels like this type of gamer makes up a lot of the US market, they're going to game on the platform that best caters to their style of gaming. A lot of FPS-centric gamers tell me they prefer the XBox controller over the DualShock. In addition, online and the multiplayer experience through XBox Live, as well as timed exclusives for many FPS titles, make XBox a natural choice for them.
XBox recognizes this core demographic, and thus many of their core games are shooters. It's a cycle. XBox also realizes that the American market is their stronghold, and thus has tried to tap into the large casual market with Kinect.
With a more diverse core lineup, the PlayStation brand does better across a diverse worldwide gaming demographic.
Just my 2 cents (PlayStation gamer of all genres)
