According to the bloggosphere there is a shortage of tanks. Others have been talking about how tanks are choosing not to queue because it is harder, but a point has been missed.
We are getting a large buff if we run with 3 strangers. 3 strangers means no guild XP or rep.
Take your choice - Guild XP on a guild run, or bonus damage with a random group. Even if your guild group does not know how to run the dungeon, you can learn, and feel like you are progressing. With pugs, there is no such feeling. I am a tank, and know my preference. If you were a tank, what would you prefer?