Imagine you're writing an epic tale that gets converted into the world's most popular TV show, then flown into the side of a mountain by some shitty director. I'd imagine the hate mail and public ridicule he probably got would make anyone lose the drive to finish.
What if the ending to show is actually really, really similar to the ending of the books? I've seen theories floated about how certain book-only events might make it more satisfying even if the broad strokes (Bran becoming king etc) are the same - but damn seeing people completely reject your series conclusion would take the wind out of anyone's sails.