Of course, that they can get away with this is made possible by the increasingly cloistered nature of academic historical writing, which has opened up a discursive space for historians who write for the general public rather than just for each other. Gordon Wood explains the evolution of this growing (and in my view lamentable) separation:
Independent scholars such as Chernow, David McCullough, Walter Isaacson, Jon This gap between popular and academic historians has probably existed since the beginning of scientific history-writing at the end of the nineteenth century, but it has considerably widened over the past half-century or so. During the 1950s academic historians with Ph.D.s and university appointments, such as Richard Hofstadter, Samuel Eliot Morison, Arthur Schlesinger Jr., Allan Nevins, Eric F. Goldman, Daniel Boorstin, and C. Vann Woodward, wrote simultaneously for both their fellow academicians and educated general readers.
This is normally no longer possible. Academic historians now write almost exclusively for one another and focus on the issues and debates within the discipline. Their limited readership—many history monographs sell fewer than a thousand copies—is not due principally to poor writing, as is usually thought; it is due instead to the kinds of specialized problems these monographs are trying to solve. Since, like papers in physics or chemistry, these books focus on narrow subjects and build upon one another, their writers usually presume that readers will have read the earlier books on the same subject; that is, they will possess some prior specialized knowledge that will enable them to participate in the conversations and debates that historians have among themselves. This is why most historical monographs are often difficult for general readers to read; new or innocent readers often have to educate themselves in the historiography of the subject before they can begin to make sense of many of these monographs.
So advising academic historians that they have to write more stimulating prose if they want to enlarge their readership misses the point. It is not heavy and difficult prose that limits their readers; it is rather the specialized subjects they choose to write about and their conception of their readership as fellow historians engaged in an accumulative science.
The problem at present is that the monographs have become so numerous and so refined and so specialized that most academic historians have tended to throw up their hands at the possibility of synthesizing all these studies, of bringing them together in comprehensive narratives. Thus the academics have generally left narrative history-writing to the nonacademic historians and independent scholars who unfortunately often write without much concern for or much knowledge of the extensive monographic literature that exists.