Although academics are accustomed to writing articles and books, they much less frequently write textbooks. When they do, they likely find it much harder to do well than they ever would have imagined. This difficulty is likely to surprise them, because they have considerable experience in writing research articles and in teaching. I argue in this article that the reason that textbook writing is so hard for academics is that there is substantial negative transfer from professional academic writing to textbook writing. That is, experience in academic writing often interferes with textbook writing. Moreover, there are a number of factors that make textbook writing simply hard to do. I enumerate the reasons why this is the case.