Tolkien was a Catholic, for whom the basic issues of life were not in question, because the Church had all the answers. So nowhere in 'The Lord of the Rings' is there a moment's doubt about those big questions. No-one is in any doubt about what's good or bad; everyone knows where the good is, and what to do about the bad. Enormous as it is, TLOTR is consequently trivial. Narnia, on the other hand, is the work of a Protestant - and an Ulster Protestant at that, for whom the individual interaction with the Bible and with God was a matter of daily struggle and endless moral questioning. That's the Protestant tradition. So in Narnia the big questions are urgent and compelling and vital: is there a God? Who is it? How can I recognise him? What must I do to be good? I profoundly disagree with the answers that Lewis offers - in fact, as I say, I detest them - but Narnia is a work of serious religious engagement in a way that TLOTR could never be.
From an oldish interview here.