Multiple reasons really.
1. It is constantly being talked about, which really gets annoying. I'm considered a nerd, and a lot of people come up to me and say "You should play CoD, you'd be pro at 360 noscopes and shit". It gets on my nerves as they fail to understand that CoD is not my type of game, and 360 noscopes don't interest me at all.
2. It affects the entire medium. RPGs, RTS, other FPS - every type of game in existence will usually have some sort of change applied to it, often with the unstated [Or in some cases stated] goal of appealing to the CoD audience. Look at a lot of AAA games coming out these days. Tell me how many are pushing forward with more complexity, trying to get more deep and satisfying gameplay. Now tell me how many are trying to make their gameplay more fast paced, filled with action sequences and set pieces, "Push a button, something awesome has to happen" gameplay and increasing multiplayer focus?
Directly and indirectly it influences the direction of the medium as a whole, especially in the AAA market. I don't blame it for all streamlining - some was bound to happen anyway. However, its influence on the market is undeniable.
3. Its pricing is ridiculous. I went to the store yesterday. MW3? $99. Skyrim? $40. That is ridiculous. There is no excuse for that. Worse still, people will still buy it. What does this tell companies? Raising the prices of games is fine, as they'll still buy them.
In addition, no, CoD is NOT innovative. Small gameplay changes from issue to issue is not necessarily bringing anything new to the medium. The closest I think its come would be with those strike packages or whatever CoD calls its class system in MW3, and even that's not highly innovative.
In addition, the attitude displayed in that post has added something more to my list:
4. The idea that an expansion pack is worth $99 [Or $60 in other countries]. Now, you state there are small changes between the CoD games. I will agree with you. However, you also state that nobody really buys it for the Single Player.
So, what people are buying it for is the Multiplayer, correct?
Now. What is new in the multiplayer each iteration? A few minor UI and balance changes? A new perk or two, or a new weapon? A few new maps?
Is that really worth $60? IMO, that's worth $20, $30 at most. Remember those days when you would get an expansion to a game, and it would have new maps, new races/skills/classes/[insert genre dependent thing here], new mechanics, better AI, a continuation of the story, a new game mode and a few extra goodies for a lower price than a new game?
"Yes. Thems was good times" [God I love that game XD].