Because the movie trilogy was extremely successful and is associated with great production value by a whole generation?
Don’t get me wrong I wish Amazon had the courage to just put 500 millions of dollar on completely new IP but that’s just not how American entertainment companies work.
For Lord of the rings, it's also wise generational timing (akin to Star Wars episodes 1 and 7).
The Fellowship of the Ring (film, 2001) is just over 20 years old, which creates, now, 25-45 year olds significant childhood nostalgia, just in time for them to be able to show their ~5-15 year old children (in their most formative years)
Don’t get me wrong I wish Amazon had the courage to just put 500 millions of dollar on completely new IP but that’s just not how American entertainment companies work.