the chronicles of narnia - Why was Susan treated so unkindly?

Warning: Narnia spoilers ahead! And also much, much text.

At the end of chapter 12 in The Last Battle, the final installment of C.S. Lewis’ Chronicles of Narnia, we are told that,

“My sister Susan,” answered Peter shortly and gravely, “is no longer a friend of Narnia.”

“Yes,” said Eustace, “and whenever you’ve tried to get her to come and talk about Narnia or do anything about Narnia, she says ‘What wonderful memories you have! Fancy your still thinking about all those funny games we used to play when we were children.’”

“Oh Susan!” said Jill, “she’s interested in nothing now-a-days except nylons and lipstick and invitations. She always was a jolly sight too keen on being grown-up.”

“Grown-up, indeed,” said the Lady Polly. “I wish she would grow up. She wasted all her school time wanting to be the age she is now, and she’ll waste all the rest of her life trying to stay that age. Her whole idea is to race on to the silliest time of one’s life as quick as she can and then stop there as long as she can.”

So clearly, in a rather Peter Pan-like way, Susan had ‘grown up’ and no longer believed in Narnia. She had, at some point, shut off her ability to believe (like Digory’s uncle did originally) and had locked herself in our world.

Now, I’ve spent the past few months intermittently reading the Chronicles of Narnia for the first time, and I don’t recall coming across any parts where this is really explained in detail. In Prince Caspian, there’s the bit about Aslan guiding the Pevensies, with Lucy being the first to see him and Susan being the last and most hesitant one—but she does eventually come around and regains her faith:

“Lucy,” said Susan in a very small voice.

“Yes?” said Lucy.

“I see him now. I’m sorry.”

“That’s all right.”

“But I’ve been far worse than you know. I really believed it was him—he, I mean—yesterday. When he warned us not to go down to the fir wood. And I really believed it was him to-night, when you woke us up. I mean, deep down inside. Or I could have, if I’d let myself. But I just wanted to get out of the woods and—and—oh, I don’t know. And what ever am I to say to him?”

“Perhaps you won’t need to say much,” suggested Lucy.

[…]

Then, after an awful pause, the deep voice said, “Susan.” Susan made no answer but the others thought she was crying. “You have listened to fears, child,” said Aslan. “Come, let me breathe on you. Forget them. Are you brave again?”

“A little, Aslan,” said Susan.

Maybe my memory is just bad, but I don’t recall any mention after that that Susan had lost her faith, until that rather negative description of her in The Last Battle. At this point in the book, I was rather expecting another turn-around for her—I had wondered why she wasn’t there and assumed that she would somehow be reintroduced to the story.

But she wasn’t. She is not mentioned again at all.

Towards the very end of the book, we find out that the jolt that brought Peter, Edmund, Lucy, Eustace, Jill, Digory, and Polly back to Narnia one last time (and eventually to, erm, ‘Super-Narnia’ for lack of a better name) was in fact a train crash. In our world, all seven of them died, and they will therefore spend their afterlife (or whatever we want to call it) in Super-Narnia.

That leaves Susan.

Seen from Susan’s point of view, she essentially loses all her siblings, some of her closest childhood friends, and her parents (if Edmund is correct that they were on the same train going down to Bristol) in a terrible accident. She never finds out that they (not counting the parents) are taken back to Narnia—as far as she knows, they just die and she’s left behind as the only survivor. The only person in the world who knows of the existence of Narnia, in fact.

Even in Super-Narnia, none of the others seem to spare a thought for their sister and friend whom they now know they will never see again.

That seems to me rather a tough deal for Susan. Lewis must somehow have considered Susan particularly unworthy to exclude her like that. In fact, in the entire series, I cannot think of a single other example of an essentially good character going ‘bad’ and then not eventually repenting and being forgiven for it. Edmund in The Lion, the Witch, and the Wardrobe, Eustace in The Voyage of the Dawn Treader, etc.—all misguided betrayals, and all forgiven and redeemed.

But not Susan. Susan is, in religious terms, kept in purgatory and out of paradise permanently. And there isn’t even any real mention anywhere of what she did to earn this tragic fate without even the opportunity to repent and be forgiven.

What (either in or out of universe) was the motive for giving Susan such a hard lot without even explaining what led to it?