Final Fantasy is a wonderful series, and currently it is only being released on PS3/PS4 due to Microsoft's policy of not allowing games to connect to additional servers, that our outside of XBOX Live. Since what makes Final Fantasy XIV so great is the volume of people playing the game, it would hurt the game to only have servers setup for XBOX players.
Square-Enix wants to bring XIV to as many platforms as possible, and having it release on the XBOX One will be a great addition to the lineup. However Microsoft's policies are the main factor into getting this game on the new XBOX.
The Producer/Director "Yoshi-P" had the following to say:
"The policy has not changed on Microsoft's side; the main reason from our side is that I don't want the community to be divided; to be split into two or more. For example, one player might be on the PC version, another might be on the PS4 version, and I'm playing the Xbox version - but we're not able to join the same game servers. That is just... I just don't like the idea. I disagree with it."
"I would love for as many players as possible to enjoy the game worldwide, so from a business point of view it could make sense to have it on the Xbox platforms. But as I mentioned earlier, that would bring about the tragedy that somebody might have to give up their existing character. Then players might get frustrated, and then they ask us - 'Square Enix, can't you do something?' and then... yeah. Unless that happens, we can't really do anything."

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