The Reason Why GRRM Didn't Finish the Books
"...“We did not need anything else than Winds of Winter and the last book,” one commenter wrote. “All fucking spinoffs and histories and other moneymaking stuff could have waited!!!!!”
"Martin kept his head responding. “I am not sure HBO would agree that the spinoffs could have waited. With GOT set to end in 2019, they put five of them in the works, so as to have a new show… or more than one… to take up the mantle in 2020,” Martin wrote. “The successor shows were going to happen regardless. I prefer that they happen with my participation and guidance, rather than without it.”"
My interpretation is that HBO wanted to make money. HBO couldn't do any spinoffs without GRRM's permission so he could've made them wait. It sounds like HBO and GRRM placed making more money ahead of finishing the GOT books which hurt the quality of the series in the last two seasons.