I just don't understand though why they couldn't find good writers considering the time, popularity, and money available.
That's Hollywood baby.
TLDR - DnD are bad at their job without a more creative person to draw the outlines for them to color in.
The more detailed answer is hard to say without being a part of that writers room and that development process. I can give you a ton of insight into the many writers rooms I have been a participant in and make a ton of educated guesses, but at the end of the day it comes down to DnD being
The Showrunners of the GoT TV series and every single major choice falls to them to either make on their own or be enough on top of their pipeline and narrative to assign to someone they trust to make.
Looking at a few wiki's and and articles online, I can surmise that they basically had a few writers room assistants/development assistants/story editors/script supervisors (Dave HIll, Bryan Cogman) in the room that might have either done some great stuff, or some harm. Like I said, without being actually in that room and a part of that process, it's impossible to say just how they managed to fuck up so badly.
Generally (and also anecdotally since this is only from my own experience), the showrunners I work with do everything they can to actually figure out their ending and then backtrack the best possible way to get there. Before you even start thinking about cast or specific writers you have walls and walls of notes and string not unlike the Pepe Silva scene in IASIP. That process usually falls apart for outside reasons. Those being that actors, directors and writers you used or like quit/die/demand more money and you scramble which starts a failure cascade that continues indefinitely...OR, you realize you don't actually have the best original narrative writing skills from the outset and want to simply try a different route. That is what i would guess happened here.
DnD are REALLY good at being adaptive writers. We can see that from the first 4 seasons. They had a structure in place and all they had to do was color by number for the most part and sprinkle in their own "creative juices" when their egos demanded it and it worked. But once that outside narrative structure is removed, all you are left with is that "creative juice" fueled by ego and in this case, it came from two guys that simply do not have the chops to write good original narrative fantasy fiction for television. They lack that skillset.
Why couldn't they just hire someone good? Ego and The Process.
Ego because these guys were coming off a MONSTER show for 4 (maybe 5) seasons of some AMAZING television and probably thought, "Hey, we got the notes from the Fat Man. We are crushing this." and then didn't even entertain the idea that they are just not as good at macro storytelling (using a million micro stories) as GRRM (hate him all you want but he is a master at exactly that). So off they go with their $100m season and the ratings stay up. The hype stays up and they do the same thing they did.
The Process because hiring a writer (or at least a good fit for a specific project) is not something you can do in a week, or even a month with any hope you are going to get good results. I read a few dozen scripts/books/shorts a week ranging from theater, TV spec, feature spec, adaptations and even fanfic and trying to find someone that can convey the exact tone you are looking for and who has the actual writing chops to maintain a cohesive narrative is actually pretty fucking tough and goes through this whole process of interviews, sample writing, agent phone calls and their busy schedule. And we STILL fuck it up quite a bit. That's why DnD probably just hired people from within simply because "they were there and 'get it'". Sure they could have simply thrown out an offer to people like Steve DeKnight or Ron Moore but that just goes right back to Ego.
They thought they had it in the bag.
NARRATOR
They didn't.
