Allegedly, the version released is quite a bit behind the updates that the Japanese version currently has. Furthermore, the game requires an account registration to play, whereas in Japan Xover can use services like Facebook and Twitter to log in. It is also region locked, and you cannot add Japanese players to your friends list.
Originally, Capcom of America intended to release Xover in the west as well, but those plans were indefinitely put on hold; probably due to the poor reaction the game received. Nevertheless, there are those who want to play it, and the Korean release could be a sign that it's on its way.
Thanks for the info, Django!