Peter Jackson has confirmed that The Hobbit: The Battle of the Five Armies will be the last JRR Tolkien adaptation. The Hobbit and The Lord of the Rings were sold by Professor Tolkien in the late ’60s, the film rights,” Jackson explained in a press conference cited by The Independent. “But they are the only two works of his that have been sold. So without the cooperation of the Tolkien estate, there can’t be more films.”
The Tolkien estate has long loved the money the films have brought in but have hated the films themselves for not being slavishly faithful enough to the books (which having read all of them can only think "thank god for that") and really hated the merchandising overkill that followed (can't blame them for that). Tolkien’s son Christopher told the French newspaper Le Monde in 2012, “Tolkien has become a monster, devoured by his own popularity and absorbed by the absurdity of our time...The commercialization has reduced the aesthetic and philosophical impact of the creation to nothing.” Which of course is nonsense since the books remains untouched and can continue to be enjoyed for generations outside of the movies. Never got why people think one somehow weakens the other.
I suspect that really all the hate is about money. If Warner Bros offered enough money for to adapt the rest of Tolkien's works, the estate would probably agree. However, what remains is really esoteric, detailed Middle-Earth world building that Tolkien fans love but for the average movie goer (most of who have not and will not ever read the books) it would come across as silly and pointless even assuming a reasonable adaptation was possible.