Why? Microsoft wants to try to contradict the amount of games and DLC released sooner on the PS3 (their competition) then on their console. For example: Assassins Creed: Brotherhood had exclusive content that was released on the PS3, while the Xbox 360 had early access to add-ons for games like Call of Duty: Black Ops. Some may recognize this tactic being used by Microsoft as it was used for Dead Money, the first Fallout: New Vegas DLC.
The DLC has been announced before the game, for those who don't know, and all who haven't are enthused to check out more about The Elder Scrolls V: Skyrim. The game is set to be released on the 11th of November on Xbox 360, PlayStation 3, and PC.
Don't forget to check out all our Skyrim coverage over at The Elder Scrolls Wiki