The same reason as people were obbsessed with Modern Warfare, 2 years ago. Then Modern Warfare 2, a few months later. Now there's Black Ops, so it's time to get addicted to a new, but hardly different game.
Though some people don't like the idea of killing, death appeals to some people. Some people like the game and some people don't, it's all up to personal preference. I like the game called Blazing anges and It's about fighter pilots in WWII, but other people may not find the game so fun.