That and the whole there being three other books worth of material that the series just noped the fuck out of because amazon didn't want to keep making the series. They did the same thing with The Wheel of Time.
There's a 30 year gap in between the last episode and when the story starts again. That's a perfect place to end the show, and then pick it back up down the road.
Actor contracts etc the longer a show goes on($$), and the time lapse between book 6 and 7. Still would be more than amazing to see on screen. And if one has seen the show and not read the books I cannot recommend them highly enough.