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

Apart from the particular strengths of the camera itself, a very important factor in the sales success of the OM-D is that there is a large number of OM shooters from the '70s, '80s and '90s who have been waiting for an Olymous like this.  Panasonic has no such legacy to draw from.  Had the exact same camera been built and sold by Panasonic with their name on it, it probably would sell well, but not in the same numbers as it has as an Olympus.

Call it brand loyalty, fanboyism or the rosy glow of fond memories, but it creates an instant market and the OM-D was ideally positioned to tap into it -- a lesson they learned with the Pens.