That's what happens when you start doing a movie/show based on a novel series that is still not done yet. You get the hype of a popular book series that hasn't finished yet, but you have to eat the risk that the movie/show will overtake the book in terms to story in a few years.
That constantly happens in Japanese anime. Show writers have to constantly write crappy filler episodes that has nothing to do with the books, until the books have enough new material for the show to animate. Filler has be known to get shows canceled.
Obviously you can't do that with GOT.