What Does It Mean to Write a Popular Book?

Most scholars write books and articles for other scholars, using jargon and presupposing knowledge available only to experts trained in their discipline. But some scholars write books designed for popular audiences about their fields of expertise. Can non-scholars write books like that? Should they try? Why do most scholars choose not to do so? Are there pitfalls in trying to communicate complex knowledge in simple terms? Is it possible to do so without "dumbing it down"? And why do so many academics look down on fellow-scholars who try to do so?