Final Fantasy XI isn't just one of the only console MMOs in existence, it's also the most successful of the lot, due in part to the wide spread of available platforms. Since the game has long been playable on the Xbox, there were several Final Fantasy XIV fans (or Xbox 360 owners) hoping it would eventually come out on that console as well. But it's not happening, and according to a recently translated interview from 4players.de, the reason rests squarely with Microsoft.
When asked in an interview why Final Fantasy XIV wasn't appearing on Microsoft's console, Hiromichi Tanaka explained that Microsoft was unwilling to work with Square-Enix to deal with the Xbox Live setup. As he put it, it seemed the company made a one-time exception with Final Fantasy XI to get the console in people's minds as an online platform, but now that it's established the gates are closed. There are currently no plans to adapt the game for the console, but it will still be coming out for the PC in late September, with a Playstation 3 release due at some point after that. Check the interview here (if you can read German), and the translated highlights can be found here.