Most nonfiction books meander around and belabor their central points. Most nonfiction writers aren’t particularly skilled at coherent and compelling structure or rhythm in prose and there’s only so much an editor can do. Quite different from fiction writing, good nonfiction requires a certain continuity of thought and mastery of distillation.
Instead of plowing through hundreds of pages to express a simple thesis with a few supporting themes, most nonfiction authors would be much better served by writing a compact and well-structured essay to express their thesis in the starkest possible fashion. When ideas stand alone it’s not only easier for the reader to digest and analyze them, but significantly easier for the author to express them accurately.