Hmm, I'm not entirely convinced. OK, Game of Thrones had good word of mouth, but it was also very heavily promoted and trailered using a very wide range of media. And GRRM as well as being a good author has a superb grasp of the fine art of engaging large numbers of people and making them feel like almost-personal friends, and he works very hard at it.

OK, GOT was on an obscure channel, but a lot of resources went behind getting people to that channel from other places that are not obscure, and the word of mouth fed off that as much as vice versa. Dirk Gently had none of those advantages (and was of course also produced on a much lower budget).