Maybe because it's such a great game ?
Maybe because FPS mode does change the gameplay a lot ?
Maybe because some people didn't play it last year ?
Maybe because it's an amazing gift to buy for the holdays ?
Maybe because they are not yet lots of great games on the new-gen systems ?
See, there are reasons for those millions sales.
I bought it least year on PS3. I will happily buy it in january for PC.