Why did the series get a following? Well, it established itself as the best WWII FPS franchse, when WWII was THE in thing for FPS, because it got away from the entire space marines thing (while still acting the way Halo did with life regeneration). Then, Infinity Ward decided to go modern, upped the story and production for COD4: Modern Warfare (Single player), and threw in an RPG-grinding experience where you got XPs for fighting and perks. RPGs have an addictive quality and Modern Warfare had it. Then, they announce Modern Warfare 2, and promise to up it even further, and got the production mill going.
With Modern Warfare, they captured what players wanted, modern combat. And throw in the "tipping point" of everyone's friends playing it, and there you go. It tipped the way Halo had.
