That's because they aren't stupid enough to get rid of such a great game. You don't see that often. No, the gamestop only exclusivity excuse doesn't work. The game has sold much better than expected.
Buy it because I said it's good. No one wants to listen to me talk about it's top knotch soundtrack, refined gameplay, easy to play while maintaining complexity factor, outstanding and original world design, good english voice acting, its hundreds of sidequests, huge world, or great story.