Is this an example of exogesis, or isogesis?
Are you trying to discover the author's original intent, or are you trying to cram some sort of meaning, or political agenda into a strory that had no such aim?
I've never heard of such symbolism in the Wizard; it may have been the author's intent, but this is the first I've heard of it. Please research and tell me more.
As to trying to explain its popularity.....Its a good story, well told, where Good triumphs over Evil.