The full report (translated to English) reads:
“Regarding software for Xbox360 version of “Metal Gear Rising: Revengeance”: Due to various reasons, the launch of the Japanese version will be only allowed to PlayStation 3. Customers who have been looking forward to the launch for the Xbox 360, as well as to everyone involved, I sincerely apologize for the inconvenience. I Looking for your understanding and to be humbly pardoned.”
It’s not hard to see why this the case. The XBox 360 doesn’t have a great reputation in Japan…at all. Many games released for the US and EU markets hardly make a dent in Japan, and Microsoft lacks many of the Japanese developers and exclusives that Sony has. It may be disappointing for whoever picked up a 360 in Japan (you know who you are), but it is what it is, and Kojima Productions made the smart call.