The problem is that throwing money at it is a huge risk. The audience been losing interest in Middle Earth throughout the years (just look at the box office gross and critical response to the Hobbit films) and there is no longer a widely recognized IP left from Middle Earth, so there is no...
I doubt any more movies will be made any time soon, the Tolkien Estate owns the film and merchandising rights to the rest of Tolkien's books from what I understand and they don't have a great history with the production companies that have made the movies so far.