All these answers are from people who heard a half assed theory online and are just regurgitating it. The truth is Microsoft has never really fought for a good amount of 3rd party games to be on Xbox and this was just one of many to skip the console.
Because a lot of people were vocal about what an idiotic move it was not to push for this game, Microsoft decided to request a version for their console. Unfortunately, because they waited too long to come to their senses, the Xbox version will not be available until next year. Fortunately the Capcom Fighting Collection 2 was also announced to be released on Xbox as well.