Maybe they just ran out of ideas? These two didn't even write the original stories; the author George RR Martin did, but he didn't finish the story before they got to season 7 or 8, so they made up all the last stuff themselves without him.
Even if they had written the screenplays themselves, that's no guarantee of future quality. I can think of two cases with movies where the same people made horrible sequels: the original "Highlander" was great, for example, but the same director (and story writer) did the sequel and it was horrible. Similarly, "The Matrix" was revolutionary, but the same two brothers who wrote the story and directed did the sequels, and they weren't that great (though arguably not the huge drop in quality between Highlander 1 and 2).
The quality of Game of Thrones directly maps to how much source material they had to adapt.
The show runners have taken all the heat, but the fact is GRRM knew he needed to wrap up the novels before the show caught up. He wrote many blog posts about it. Unfortunately, it’s clear now that he never will.
Expecting the show runners to finish a series that even the original author can’t is just unreasonable. To that point they had writing credits on The Wolverine and a couple other shows.
But according to popular internet sentiment, Benoif and Weiss are to blame, not HBO or GRRM for leaving them high and dry…
It highlights what a good screenwriter Martin is. He writes for the screen in his novels (and this is good) so much so that it doesn't engage with my imagination at all which is actually what you need for television!
Obviously he has a good imagination and it's this that the other writers couldn't replicate. Maybe the imaginary is harder to capture in a screenplay compared to literature. Fascinating topic.
Even if they had written the screenplays themselves, that's no guarantee of future quality. I can think of two cases with movies where the same people made horrible sequels: the original "Highlander" was great, for example, but the same director (and story writer) did the sequel and it was horrible. Similarly, "The Matrix" was revolutionary, but the same two brothers who wrote the story and directed did the sequels, and they weren't that great (though arguably not the huge drop in quality between Highlander 1 and 2).