Pokemon is a rather unique beast. Pokemon is not just a single game that has been hashed over and over, but a multi-leveled-media machine. There's your main line games on the portables, cards, spin off party/camera/pet games, colosseum, cartoons, merchandising, music, costumes, competitions and more. You don't have to be a gamer to enjoy Pokemon. If all you enjoy about it are the cartoons, that is ok. They would like you to buy some of their other stuff, but there is no hard sell. If you leave and come back, it will always welcome you back no matter the level of participation, even if you just like Pikachu.
Madden and it's ilk are also one offs. Due to the lack of any direct competition due to licensing and being part of another machine it can get away with what are essentially roster updates year after year as long as they never, ever offer DLC with roster updates. Also everyone "knows" there is another out next year and are trained via perverse peer pressure to get the current game so they can play with their friends. Along with them cutting server access early, "Project $10", makes it a very strong are you in or your out situation, coupled with the absurd fixation on American football that cumulates into the Super Bowl, makes Madden work. EA has a total monopoly on "Sports" and this is reflected in the yearly releases.
COD, is well, COD. That's it, well that and a lot of USAUSAUSAUSA!. There is no supporting infrastructure, no viable long term plan(Make more COD not now, but yesterday), no super advantage they can use other than this is COD. Activision doesn't have the monopoly on war or FPS. They hit gold with Modern Warfare after only digging up silver for years and has proceeded to demolish the mountain in order to quickly fill in quarterly reports. Expensive player base splitting map packs, bad server support, poor QA, glacial response to complaints, developer real-politics, all cheap and profitable in the short run, draw down on good will that got banked with MW1. One bad game they can't hide under explosions and it's done. They have already gutted their own talent when they put their hand up to say excuse me, "I need to go to the bathroom" with their original dream team, so no doubt they have already started grinding down who ever is left.