Reason 1: The destiny crew had to change a lot of story last minute when a writer dropped out. This could have caused there to be no story tie to mercury and thus no reason to go there. Reason 2: Mercury wasn't fully developed and ready to used in Destiny. It could be being worked on now to be released as DLC Reason 3: The game has features and areas locked away for future use. Not only can Bungie sell them as DLC but it allows them time to fix the problems they have with the current features already available. If the had released everything at launch the game would have flopped due to all the bugs. Reason 4: perhaps it is/was already to go but Activision, being the greedy *ssholes that they are, decided to scheme us for more money by intentionally cutting it out of the base game to market as add-on content. Will we get Mercury? Probably. Will we have to pay for it? Probably. *sigh*