Obvious reasons:

-Better graphics

-Higher resolutions

-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.