Microsoft bought this “timed exclusivity” of the content from Activision for an undisclosed amount, giving their users more than 1 month's headstart in which to enjoy the Stimulus Package before other consoles got it. A new marketing tactic, this certainly gives Xbox 360 users a feeling of getting an “ahead-of-time” release, while other platform users will feel that they are getting “dated” content.
In the two weeks since its 30th March release, the DLC expansion has been downloaded by over 2 million Xbox 360 users. As you might remember, the launch was beset by numerous problems for users, caused by Microsoft releasing the game patch in the wrong sequence, i.e., after the DLC.
This sort of error will probably not happen again, and PS3 and PC users should have an easy time about playing the new content.