this makes me sad

This has been bothering me for awhile. The Chronicles of Narnia are symbolic of Christ, heaven, and lots of things in the Bible. And we all know "Once a king or queen of Narnia, always a king or queen." So, what about Susan? She was a queen of Narnia, and reigned for around 15 Narnian years, I think. Yet, in The Last Battle, Peter said to Tirian, "My sister Susan"... "is no longer a friend of Narnia."