I guess George R. Martin has lots of time to practice his monster fighting techniques with all the writing he isn't doing.
Reminds me of an old boxing quote: "“It's hard to get out of bed in the morning to go for a run when you're sleeping in silk sheets”. It's probably a lack of motivation as he has no real incentive (at least a financial one) to write them. He's probably just sitting back and enjoying the fruits of his labor. It's a shame though, I would have liked to have known how it really ends. He could have fixed the debacle that was season 8. He could have potentially rivaled Tolkien, but only if he finishes his masterpiece.
There used to be a bookshop in Sydney that specialised in science fiction and fantasy. They would often have visits from well-known authors to talk to fans and sign their works. I had quite a few books by English writer Patrick Tilley, in particular "The Amtrak Wars". At this stage the story was still unfinished. I had four books. Eventually there were six books in the series. Chatting with Patrick about his stories I questioned when the next book was coming out. Imagine my disappointment when he said it might be a while as he had lost interest in the story............
the masterpiece were the first 3 books. the last 2 ones were already going downhill. Too much sidestories, too many new minor povs, still good ideas but in less quantity and drowned in much more content.