Quite simply, xbox 360s cater to the lowest denomination of gamer; a gamepad means game interfaces have to be simplified, and make it easier for the kiddies to use their gamepads. The xbox 360 is lagging behind PC hardware type, thus, the graphics tend to be inferior when it's a cross platform game. Similarly, the xbox 360 caters to people who don't know anything, and that's alot of people, thus, devs/Pubs like Ubisoft develop a nicely optimized game for the xbox 360, and a piece of unoptimized garbage that gets optimized a couple patches later.
The xbox 360 encourages stagnation, because the hardware and gamepad constrains real gameplay and graphical innovation---there's a reason games like Crysis can't be ported in their real form to the xbox 360.
Call of Duty 4 suffers the same thing, the multiplayer will only be 16 player, because that's stretching the xbox 360's hardware--or at least that's what IW would say. The huge playerbase of the xbox 360 is a bunch of retarded n00bs, so the devs have to simplify their games because these little craps have an attention of span of 5 seconds.
So there you have it, I feel sad I had to explain it to you.