-More precise controls
-More mature player base (less trolling and useless chatter)
Reasons you may not know about:
-Multi-monitor support (Nvidia Surround, Eyefinity)
-Mods (user-created content, maps, missions, new game modes, etc)
-Quicker integration between other apps (such as YouTube, Facebook, official forums, etc)
Reasons why the other versions of MW3 are popular:
-Quick, dirty, and cheap.
-Just like your mothers.