My guess? He's sick of GoT but can't drop it cause it's a seriously nice source of income but given the oppurtunity i'm sure he would write about something else.
But could he though? I mean, come on, he's not that fantastic as a writer, to put it nicely, so maybe he knows that he can't do anything else. Oh sure he would sell, at least at first, just thanks to his name. (as Rowling did)
But on the long run? No, he's just going to keep it up like that, finding more excuses to keep pushing back the release of the next book for some more years. Gotta keep the cow alive cause once GoT is done, he's done.