It’s one thing to write a textbook; it is another matter entirely to get a critical mass of people to buy. For someone socialized as an academic, the audience for a textbook is a far less specialized one than one is accustomed to addressing. The format has to be perceived as accessible; the audience has to find it welcoming.
A website about trade books provides analytics enough to give a would-be author pause. According to Bookscan, of the 3.2 million books tracked in 2021, fewer than 1percent sold more than 5,000 copies. While I confess to find myself turning to book writing for the sheer love of the spaciousness it affords to expand on ideas, it’s hardly a wise investment of all-too limited time to a write a book that only a few will ever buy.