I don’t think they actually do sell that well. Books by talking heads sell well, but they have a dedicated following. Outside of the big time pundits though you don’t really see that many political books on the best seller list, and even in that group it’s usually one big rush to buy. A year later they’re in the remainder bin.
So maybe the answer is, it’s not political books it’s political figures. Their supporters buy the books.
But Morris's books drop off after a few weeks and never come back. Most of my books are that way: "Seven Events that Made America America" and "48 Liberal Lies" continue to sell well, but the star, of course, if "Patriot's History of the U.S.," now with over a half million in print and still going strong.