As a long time fan of the books, I've recently been thinking a lot about how and why the quality has downturned since the show outstripped the books. Also why shows based on books are so much better than their traditional TV as first in media counterparts.
I think there are a few things that come into play:
Budget - As @Kenny mentioned the showrunners are budget constrained, and have to make decisions based on that, not just on what best serves the story. Since at this point the story is not set in stone, they can adjust the story to match.
Actor contracts and fan outrage - When you are the one rolling the story forward with characters that people like, it's very difficult to make a decision to kill off a character, especially a main character like one of the Starks or Lannisters. You have to consider that you are firing the actor and any supporting staff they might have, and it is hard to take that out of the equation. You also run the risk of fan outrage and bandwagoning against the show, people upset you killed off their main character. If the death is already written in the novels, it's much easier to deal with this. You set up the actor, like Sean Bean initially, and let them know that they are only going to be around for a few seasons. Imagine in the books hadn't been written first, Sean Bean would still at this point be having flashback scenes after 8 seasons.
Time Constraints - These books have taken years each book for GRRM to figure out how to get people from one place to another, decide which characters are going to make what decision, and how how thousands of troops are getting fed at Winterfell. At the end of the day, all he is doing is moving words around a page and it takes that long. A big budget production like GoT has thousands of people and logistics involved, which means corners of the story need to be cut.
All this together leaves me really hoping that GRRM finishes the books after the series is over. I'm curious how things will be different.
By the way, if you like GoT you should check out The Expanse. It is another huge budget show which is very faithful to the source material, with the book series nearly finished already. To make a simplified comparison it is a bit like Game of Thrones: In Space.