Diamond does not ever tell us in the preface why he has chosen to write in the style that he uses. We must simply try to infer why this style would be useful to him.
First, let us characterize his style. We can say that it is an informal and not very academic style. So, why would he want to write in such a style? The reason that I would suggest is that Diamond wants the average reader to be able to enjoy his book. He does not want the book to read like a textbook because he wants it to be accessible to people who are not in academia.
I would argue, then, that Diamond chooses his style because he wants the book to appeal to a wide variety of readers.