Speaking with IGN, the reason why A Realm Reborn is missing from the Xbox platform is simple: exclusivity. Spencer notes that he can’t go into too much detail about it, but he states, “It’s business. As I’ve grow in this role, and I’ve tried to learn the third-party exclusivity thing – and you see us doing less of it now – [I’ve found] it’s not something I’m a huge fan of.”
He also talks about third-party developers and states, “Different kind of deals happen, and I know that’s part of this business, and maybe it’ll be my failing in the end. But it’s not something I specifically embrace with any deal that’s out there, whether it’s something else. It’s just not something I can explicitly talk to.”
This is pretty much what had happened with Microsoft and Capcom with Street Fighter V, in which the game landed as an exclusive for the PS4. This isn’t a timed exclusive either, meaning that for the foreseeable future, there will not be Street Fighter V for the Xbox One, and it would also seem that way for Final Fantasy XIV: A Realm Reborn.