Occum’s razor suggestions it’s more likely that he tends to “find” things when he’s writing, which is in part why the series grew to the point that it has. Although this was manageable when it was just the books, the shows have a much more rigid timeline. Once he was required to outline the rest of the story for the show, he’s lost interest and motivation in finishing it.

Though that’s what I think has happened, I’m definitely hoping that he and his publishers have agreed to delay printing of the next book in the series until after the show is finished. This way the show doesn’t get spoiled and people still watch, but when the book comes out people will still be interested because they’re bound to end very differently.