I'm surprised that I haven't found anything addressing this. I have been active on the Japanese beta for some time now, and would quite like to see it get launched. It looks like the metrics are all good: two excellents, two okays, and a needs work. Is it the one "needs work" that's holding it back? Is there a scoring system in place that decides when it's time to launch (e.g. 1 point for okay, 2 points for excellent, 8 points triggers a launch)? Is launching automated or is it at the moderators' discretion?
Update: I keep seeing this buzzword "critical mass" used quite often, but it's never defined, and sounds like a vague way of describing the requirement for a not-so-vague event (that is, a site being launched).