I could be wrong but is it not just because Susan stopped believing in Narnia? Lucy still shows up for The Last Battle.
EDIT: So do Jill and Polly! This seems a little reductive of Susan’s role in the story as an example of lack of faith and how maturing brings you to focus on your surroundings and lose your inner child.
Thank you for clarifying! My brain could only see this as an excerpt and I was not able to seperate it from the assumed rest of the sentence.