There seems to be some confusion about what the widest context is.
As of September 2012[update], HBO's programming reaches approximately 30 million pay television subscribers in the United States
As I surmised, Game of Thrones doesn't even get a majority of the HBO subscriber base. I was surprised to find that True Blood is #1. Game of Thrones wild popularity hasn't even made HBO the #1 premium cable, which turns out to be Encore. In my home, Encore is packaged with HBO. It and True Blood have kept HBO on the bill. And if the new showrunner on True Blood isn't up to it, HBO will be losing a subscriber.
As of August 27, 2012, there are an estimated 114.2 million television households in the United States....
Lowballing the number of people per household at one, Game of Thrones is getting 10%.
There also seems to be some confusion between artistic success and popular success. Vox populi tells us Game of Thrones is doing something right to entertain lots of people. I suggest sex and violence is the main sell, as is usually the case with the supposedly better writing on cable. I wasn't a medievalist but I knew enough not to swallow the Game of Thrones' society as grounded in human reality, instead of old melodramas. I suspect there series is much more compelling to people who know even less than I.