I mean, their books weren't bad, as far as I know.
To me it seems like, even more so than being bad writers, D&D simply are getting tired of making this show. They've been doing this for 7 years now, and I imagine that once they left book territory to fall back on, it didn't become any easier. The ASoIaF fans also seem a lot harsher than any other fandoms I've seen (like, there was plenty of outrage over no Tom Bombadil in the LotR trilogy, but it didn't seem comparable to the outrage over GoT's deviations from the books), so I imagine that the backlash also may have been getting to them.
They genuinely seemed to care about the books and making the adaptation good as they were starting, else GRRM would never have agreed to let them do it, but I think this is simply a case of they bit off more than they can chew, and now they have to finish it. Which is reflected in the last 2 seasons being shorter in number of episodes, as well as Season 7 feeling a lot more rushed even compared to the fairly weak seasons before it.