Or maybe because it's had a release every year since 2005 and hasn't seen innovation in 6+ years?
It's all subjective, I personally really like the COD franchise but I can see its flaws, but it introduced a lot of rpg mechanics into the fps market, like perks and weapons. It's just the extreme views that are present on the internet, in reality most people probably like it or don't, but it seems like when its brought up people either go OMFG BEST GAME EVAR!!!11 or LOL YOUR SUCH AN IDIOT FOR PLAYING COD PLAY SOMETHING LIKE PLANESCAPE LOLOL.