That is because, unlike most authors, Tolkien was actually trying to put symbolism into his work--he wasn't trying to just write a novel; he was telling an epic. Being a linguist, he recognized that, in the many retellings of an epic, small embellishments are added in order to provide side stories and submorals.
Tolkien certainly isn't the only author who has ever deliberately inserted symbolism, though. He is, however, one of the few among them who isn't painful to read. There's certainly plenty of symbolism in D.H. Lawrence, but it was put there for the express purpose of being dissected by literary critics.
