Major studio economics have changed in recent years and it is now critical that movies have a global, universal appeal.
Their major profit centres are evaporating before their eyes (back catalog tv licencing and dvd sales), along with declining domestic audiences.
In order to turn a profit the movies must be as generic and acceptable as possible: lots of children, the hero's journey and other myths, no sex or swearing, "fake" comic book violence or none at all.
Any kind of NC-17/Restricted rating is the kiss of death. Anything controversial will be a flop.
The problem is the death spiral of risk-aversion, unoriginality, and plot refinement to the point where the "movie" will be perfected and there will be nothing left to film.
Lord or the Rings is the new mythology.. the archetype of the Hero's Journey. Once you print it to film.. what's the point of retelling the story?