[Nokturnal]

I'm not certain of this, i don't play MP anymore - But i believe the only thing separating Steam users from the others is the patch.
As you say, the Japanese patch isn't official and this means Steam users haven't been auto-updated. Also i would say not many people who bought from a store have bothered to install the update yet, most will be waiting for the official version to avoid bugs and the sort.

Give it some more time, as soon as it becomes official you will find many more players. Until then, i guess you could re-install and skip the patch so you can play with others who haven't updated?