Honestly, CoD4 stands on my list of top ten video games. MW2 stands on my list of things less lovable then a brain aneurysm. It's just behind terminal colon cancer, yet just ahead of malaria.
CoD4's story was beautiful to play. The really epic parts were counter balanced by slower, more deliberate sections (like the nuke being followed up by the slow, crawling around dying part, yet prefaced by the epic "Get to the choppa" sequence.) MW2's story, to paraphrase Yahtzee a bit, smashes you in the face with a frying pan, and keeps at it, lest you realize how bored you really are.
CoD4's gameplay (in singleplayer) felt incredibly like something that a real modern war would feel like (minus the regenerating health and all that happyness). MW2's combat felt like a freaking shooting gallery nearly half the time (That section in Brazil, where you're running through the slums, and the door keep popping open with dudes shooting SMGs at you made me want to eviscerate the puppy of whoever came up with that idea. It's not skill based, it's "Play this section 15 times before you remember when the door opens".)
CoD4's multiplayer is as balanced as the game ever got. All weapons were viable on any map, the maps were balanced to allow all sorts of playstyles, and the killstreaks, oh God, the killstreaks, were rewards for doing well, like they should be. MW2's Killstreaks were rewards for camping, so once you've got three kills you find a nice little corner, get a fire going, and pitch your tent. And how much fun is it to be behind an AC130 blasting the living F*** out of everything on the ground? Tons of fun. Is it any fun at all to be the one on the ground getting blasted WHILE INSIDE THE ONLY BUILDING ON THE MAP? No, it makes you sour about life, TBH.
That's why.