Why is the EM5 so much more popular than the GH3?

It's pretty simple. EM5 has been out for a while so a lot of people are using it. The GH3 has not yet been delivered in volume, so there are not a lot of people using it. It's just math.

The GH3 is going to be shipping in bulk very quickly, now, though. People who ordered even recently in the last week are starting to get shipping confirmations. Those who preordered back in November and got their GH3s already, are almost universally thrilled with the camera. I don't know if the GH3 will sell in the same numbers as the EM5; I think it appeals to a different sort of photographer. It may have a wider appeal or narrower, I really don't know. The GH3's user base may be more or less likely to post in the DPR forums than the EM5 user base. But I think the number of EM5 owners and GH3 owners will be at least reaching something close to parity in a few months. What you see today as a large EM5 focus may no longer be true in due time.

Even still, I see plenty of mentions of the GH3 all over this board. There's a few trolls trying to stir up trouble complaining about it, but mostly the discussion has been quite positive. I see tons of people asking about lenses in the context of the GH3, in particular.