Since, in my own conviction (and that of many other Christians), if there has been anyone in our time who "believed in the fundamental truths of the Bible," Mr. Lewis surely did, your question is impossible to answer.
Pardon me for saying so, but your question is rather like, "Why is water not composed of hydrogen and oxygen?" That is; it's unanswerable because the premise is not meaningful.
Perhaps if you shared just what you yourself mean by "the fundamental truths of the Bible" additional conversation might be possible.
