The messiah narrative is the least of it. "The Matrix" had it, and is not nearly as enduring!
No, it is because Narnia echoes some part of reality as we believe it OUGHT to be. A part of reality that our culture DENIES and HIDES from whether you sit in the Pew, or the Lab.
Why is it that Narnians' dont keep pets? The answer is at the tail end of "Out of the Silent Planet". (I'll post this in a different thread). It is because, like the malacandrians, narnians taste the experience of meeting other sentient beings different and like themselves.
They dont need pets in order to know and enjoy the great chain of being, from which Man (well, Western Civilised Man at least) has severed himself and in so doing from both the earth and from heaven - from both his "sulfur" and his "mercury".
BTW, the last page of OSP also reminds us that in addition to sentient "animals" lewis also populated Narnia with sentient beings of the "subtle" body type. Creatures that can endure the vacuum of space, the fires of the sun, or the crushing pressure of Jupiter, as well as our own temperate realm- yet another "thing" that we rarely come into contact in our world ("Africanus" , and other crazies being the rare exception).
Lewis' work is rare in the literature of the West -
It is more than a mere "beast fable." The idea of non-human sentient life is anathema to western thought, to roman thought, and, regretably, to a large part of christianity- Man is at the Apex, and God is faaar above the clouds. But in Narnia, God's spirit pervades both animals and trees and even stars. That's not what most of us hear from the pulpit.
The closest thing I've seen in the last 30 years is a movie by Dreamworks about a Stallion, and the one human who could reach into his heart far deeper than Monty Roberts ever will. The one says "I will break you" the other says "great Stallion," I will win your heart. Which reality would we choose?
Or rather, which reality have we chosen? This latter question is the subject of
Wolf's Rain (japanese anime, Bones Studio 2003) It is no idle question, and the horror of the west is that too few of us (christian or otherwise) are able to properly pose the question - having become like Uncle Andrew.
As long as this cancer endures, Narnia will remain popular, until the west destroys itself or heals itself.