It's also the nerdiest show HBO has made in the last 10 years, so it has special appeal to the demographic least likely to ever pay for HBO to begin with.
Nah, True Blood is fairly mainstream. It's a paranormal romance populated entirely by good-looking people with copious sex and violence and reasonably simple plots. These are all things that mainstream viewers love, and True Blood's numbers reflect that.
Game of Thrones is a gritty medieval fantasy political drama whose viewers actually requested less sex after the first season. Game of Thrones commonly features scenes of (for example) haggard old people sitting at a table talking about whether they'll go over a bridge or whether they should walk through a forest. It is probably the geekiest show I've seen on TV in my lifetime. Maybe ST:TNG beats it, but I'm not sure.
