I believe 95% of the reason is $$$$. The reason there's so many sequels and superhero movies in Hollywood these days is that the risk factor of a flop goes down.
I suppose this is still risky if they mess it up big time, but they've already got a built in audience of tens (hundreds?) of millions. The story has been proven (obviously), so if they do it in a slightly different way while adhering to the books perhaps it's a slam dunk, according to their research.
But do they have a built in audience? I loved both the LotR books and Peter Jackson films, I love GoT and ASoIaF, I even have Amazon Prime, I probably fit perfectly into their intended target audience but I have no desire to watch this. The Jackson adaptations left me completely satisfied, a new adaptation can only go down from there.
I suppose this is still risky if they mess it up big time, but they've already got a built in audience of tens (hundreds?) of millions. The story has been proven (obviously), so if they do it in a slightly different way while adhering to the books perhaps it's a slam dunk, according to their research.