In Prince Caspian the movie, why did Aslan say that Susan and Peter would not be coming back to Narnia?

Well it's part of the plot. It has everything to do with the written story, not the actors. Aslan is basically the symbol of "God" in the book, and as such, is meant to be all-knowing. Also, Narnia is for those who believe in it. Peter and Susan are growing up and will lose their belief, and forget about Narnia.