The genesis of Middle-Earth came about as a way of making a concrete history to place his created languages in, and then just kind of took on a life of its own.
His histories and short stories of Middle-Earth can be found in not just his three novels (The Hobbit, The Lord of the Rings, and The Children of Hurin), but in The Silmarillion, The Book of Lost Tales, and The Histories of Middle-Earth edited by Christopher Tolkien.
Lewis created the World of Narnia in his novels. Tolkien set The Lord of the Rings in Middle-Earth, a completely constructed world outside of that one story.
And that is why Tolkien's theology is infused in the story in a more subtle, and yet thorough, way than Lewis could have accomplished.
Makes sense, thanks for the insight. I just finished the Lewis space trilogy (Out of the Silent Planet,Perelandra, That Hideous Strength) which was theologically very interesting. I highly recommend it if you haven’t read it. I think Tolkien’s theology permeates LOTR in a more organic, less intentional way than Lewis’. I’m not sure that makes it more thorough, but it might.