This is an obvious statement but WWII, especially the Holocaust and the firebombings and nuclear bombings, did a real fucking number on everybody, and in intellectuals the response was very much a new cheerful variant on existentialism--the key text is probably Paul Tillich's The Courage to Be. Kids had to be taught that magic wasn't real so that they could properly accept grim reality and soldier on despite it. You can kind of see this in the fact that it was The Last Battle that won the Carnegie; it's a pretty easy interpretation of that book that Narnia was a lie and the Christian afterlife is real. And of course the train crash for that extra dose of realism.