Snobs just dislike whatever's popular at the time. It happens to be CoD now, so that's what they'll bash. It's the same reason people bash, say, Justin Beiber or Twilight or Harry Potter - instead of simply not caring about things that have nothing to do with them.
As for CoD - I think it's popular for very good reasons. For once, it let us play what a lot of us truly wanted in a game. Pick up a modern day rifle and take part in a big war as it would be fought today. For me, personally, it really struck a chord. I'm so sick of using WWII era weapons, I'm so fed up with killing Nazis and frankly I don't buy "terrorists" as a convincing, menacing OpFor. Give me a TAR and a USP any day. And whether or not people would admit it, CoD did the whole modern warfare thing well. That's why it's popular.