They decided they could do better than the source material. They were wrong.

This behavior in Hollywood is something I really don't understand. Like I know there are egos out there that make stupid decisions.

But, the source material, no matter what we're talking about, is what made millions of dollars. Not some untested ideas from hack writers. The source material is tested. It works. All you have to do to excel is to take that exact source material, and translate it to a TV series. That's it.

D&D understood this (to a point), when they went to GRRM and pitched the Game of Thrones TV series. GRRM understood that Hollywood is full of these idiots, and tested them on their knowledge of the source material. When they passed, it got greenlit.

And then Hollywood acts all surprised when the show get high ratings. As soon as D&D was winging it and writing whatever crap they wanted, the whole show started to crack, until they rushed out that plot-hole-ridden, clichéd final season.