Whereas let us say Sherlock: there have been three episodes, and the fanfic writing is already going strong.
OK, why so little? There's almost no Mad Men fanfic either. I think these series are intimidating. They intimidate me. I think they both ask a lot, and conversely, a lot is already provided on screen. The reason BB fans have finally cracked and started writing is because of prolonged baiting by the show, IMHO. Man, they are asking for it!
What there is at the moment is bloody well written, and quite hardcore. Anyway, no doubt it will all take off big-time at some point, and there will be lots of romantic fluffy stuff that I won't read*, but it is quite nice to be able to feel I know the field, and I like it all. A strange and unique position.
(*ETA - I come back to read these words, and it sounds as if I am being dismissive of other people's writing, and I really don't mean to be. I just know what I am like: I prefer less romantic stories. Just my idiosyncrasy. I respect the writing that people do.)