That was the big problem. The HBO series caught up with Martin, and he was having trouble finishing his books. So the writers for the show went off on their own (supposedly consulting Martin). They were rushing, and it showed. They probably should have waiting until Martin finished his books.

In retrospect, though, we now know they couldn't have waited. Martin isn't any closer to finishing the books than before.

My guess is that he can't finish them. He doesn't know how to move from where the books are now to any kind of narratively satisfying ending, while maintaining the tone and "realism" of the books. He's written himself into a corner.

Or rather, I think it's an impossible task. Once you make the decision that your writing is going to eschew narrative conventions and subvert genre tropes, you lose the ability to move the narrative to a conventionally satisfying ending. IOW, if you stick to having your characters act the way that "real life" people would act (albeit in a fantasy setting), the plot is going to act more like "real life," rather than a storybook narrative. Meaning that it's not going to reach a point where the main individuals' or the world's storylines culminate a single spot where you have closure and a resolution of those storylines.

Put still another way, real life doesn't follow narrative structure. It doesn't come to a climax followed by a denouement. Once you've made the decision to have your characters ignore narrative structure, you can't stick to that if you want the story of your world to have narrative structure. You can do that for a single character, but not the world as a whole.