This is a fascinating question, thanks for bringing it up. I like the various points people have made. For some reason it's never particularly bothered me, and I think I always saw it in the way that I think Lewis (chiefly) intended it - as a plot device: he didn't want to keep writing about the same characters indefinitely. If Susan and Peter hadn't been prevented from returning, we would never have got to see Eustace and later Jill have their own adventures. There's also perhaps the fact that he was writing children's stories with the idea that he wanted children (and perhaps children of a certain age-range) as the main characters and since his characters were getting older, he wanted to "replace" them with younger ones.
Moving from Lewis' intentions so Aslan's, while working on my story about Susan, I started wondering how the conversation had gone that Aslan had with Susan and Peter, and decided to write a one-shot about how it might have transpired. One thing that came to me as I was thinking about it was that these stories are taking place during World War II. While the Pevensie's second adventure in Narnia had brought and end to oppression and a short-lived war in Narnia (the rightful King was on the throne and everyone had seen Aslan and believed in him again(, in our world, things were much darker and needed people who understood the truth to be there to help the world through such a dark period. A quote from my one-shot:
The Lion continued then, explaining why it [their not being allowed to return] must be so. “You children have been called to Narnia, twice now, for two reasons: for the sake of Narnia and for your own sakes. For the Narnians, that you might bring them hope and freedom. And for yourselves, that you might learn to have faith.”
After a short silence, Peter spoke up. “Faith in what, Aslan?”
Aslan looked into his eyes, as though baring into his very soul. “Not in what, son of Adam, faith in whom? You have both served me well in this world. But now you must learn to serve me in yours.”...
“Aslan," [began Susan] "does that mean…”
“I Am. You know, Susan, that I Am. You have always known this. And this is why I need both of you there now more than here. Your world is going through dark days. Some of the worst are yet to come, though a period of relief is not far off. I need men and women, boys and girls, who can stand for me in the days ahead. Narnia has been restored again, and the truth will be made known throughout the land. But in your world there is dire need of people to stand for what is right, just and true. I need you there.”
These are obviously my words, not Lewis' and very possibly not what he had in mind, but I think it works as a potential explanation for their not being allowed to return.
(If you're interested, you can read my full "conversation" here:
http://www.hedgepickle.blogspot.co.uk/2 ... g-you.html