The key words there are “layman’s terms.” In other places, these theorists refer to “everyday language” or something similar. This, I think, is one of the essential elements of these books’ (and TV series’) appeal. They talk to their audience at their audience’s level. They do not require specialized knowledge to understand, nor do they presuppose the reader’s familiarity with the regions and sites described.
By contrast, most mainstream history books published today assume the reader has a great deal of background knowledge that the reader may not have. The books also tend toward academic language and vocabulary, both because the publishing industry has made it difficult for anyone other than academics to publish and because authors feel the need to use complicated and elevated language to bolster the perceived credibility of their work. But these trends simply turn off even more of the potential audience.
I’m reading right now Vanished Kingdoms by Norman Davies, a much-hyped and history of the forgotten states of Europe (those that did not survive to the present, such as Burgundy, Aragon, etc.). The book is ambitious in scope and, theoretically, aimed at a popular audience. But the book is a mess of incoherence. Unless one already possesses at least a college level understanding of pre-modern European history, the parade of actors, historical events, and geographic locations descends into meaninglessness. Chapter after chapter lists long-forgotten battles by indistinguishable warrior monarchs without the context necessary to make sense of the story. Davies wants us to understand that no state and no government is eternal, but unless the reader already possesses a good understanding of the process of state formation in the major European powers (something, I suppose, his British audience would know somewhat better than many Americans), the lesson is lost.
This isn’t to pick on Davies. So many historians seems to suppose they are speaking to fellow historians rather than to people who might be interested but lack an expert’s command of, say, the historical development of Capetian France, or the thousand year history of the Holy Roman Empire, or the differences between the two.
The problem lies with the publishing industry, which has gutted what used to be called “middlebrow”—the books that appealed to intelligent generalists. (The exceptions, of course, are the trend of politically-motivated histories intending to demonstrate the essential correctness of some ideological point of view or another and books about modern history—if it happened in your lifetime, publishers think there is an audience for it.) The resulting polarization leads to lowbrow books, like the Dummies series, and to highbrow books appealing to scholars but to few others. The reasons for this are manifold—everything from the erosion of the reading audience, to the ceding of the middlebrow territory to cable television, to competitive pressure to sell to the lowest common denominator en masse and to rack up prestige titles in limited runs. But the end result is the same—the intelligent generalist is left to either self-educate up to the level of scholarly work, give up hope and stick with watered-down popular surveys, or turn toward “alternative” works that still pretend that non-specialists are capable of understanding and enjoying subjects too often reserved for those enclosed in the ivory tower.