They only have the rights to the hobbit and the Lord of the rings. None of the books or Tolkien’s letters that cover the second age. So they don’t have the legal rights to make it. But instead of staying there and doing the other stories and characters we don’t see the other movies. They try so make a show from books and notes they can’t do or make it look like it’s from those source material. The best way to describe it is fan fiction with a lot of money behind it.