One of these things is not like the others...

[Estelyn Telcontar]

Yes, all of those reasons do apply. My thoughts are more concerned with the role of the author and his created universe. Wouldn't it be just as boring to know everything about your own invented universe as it would be to have nothing left to discover in this world/universe?! I think Tolkien needed some mystery left for himself, to keep his own interest in Middle-earth alive.

He wrote about that in 'Leaf' by Niggle. While alive, Niggle never was able to finish his painting - there was always something more to be seen at the edges or beyond the main tree. And after his death, when he encounters the real tree, there is time to complete his vision - but when he is done there, he moves on to discover new vistas.

Even little Niggle in his old home could glimpse the Mountains far away, and they got into the borders of his picture; but what they are really like, and what lies beyond them only those can say who have climbed them.

Interestingly, C. S. Lewis ends his Narnia books with a similar concept: "Further up and further in!" There is always more to discover, not only for us readers, but for the author himself. Knowing what we do of Tolkien, we can assume that he was content to leave some mysteries unsolved even to himself.