1) It's got brand recognition so it's familiar to a bunch of people who are too scared to play anything else.
2) It has a LARGE fanbase so multiplayer games are as easy to find as a white flake in a hospital.
3) To the moronic masses who have never actually seen a soldier or a military base in real life, 'Call of Duty' is a juvenile war fantasy. It's not a matter of realism; it's a matter of being a soldier of war with a gun and being incredibly- I use the following word insincerely due to it conflicting with my actual definition of the word- badass and killing everyone. For any of you, even I will pray for your health if you actually hold such views in front of your superior officer, and I will pray doubly so if you voice those views.
The fact that the game is set in the real world doesn't matter; 'Call of Duty' fans are (I imagine) also fond of 'Mass Effect' because the gameplay is similar; hide behind walls (like a soldier), kill anything that pops out to look at you (like a soldier), proceed (like a soldier). Games like 'Serious Sam' and 'Painkiller' are in no way the same as you can jump very high, run while gunning and carry seven billion weapons and not look down the sights. Also, you're likely to get savaged by a bunch of extremely strong enemies at once (this has doomed me several times in Sam Stone's first adventure). 'Call of Duty' players don't want to be superhumans; they want to be soldiers.
I'd also like to note that YOU are not necessarily one of these people; I'm just speaking generally.