Yup. And, conveniently, wrapping up the series just as they pass the books: they have no more source material to draw from.
I was being more snarky than anything but I really do wonder how much not having the novels anymore (yes, I know that actually started this season) or not having to have to cover plotlines has to do with it. Not much, I'm sure.
Also, GRRM has mentioned that he wants HBO to do a Tales of Dunk and Egg film(s).