I don't think it's that psychology books in general don't sell well. The nonfiction bestsellers list always has a lot of psychology books in it†! But when it was "POET," it got shelved in the psychology section of bookstores. So, if you went into a bookstore looking for help coping with your childhood traumas or marriage problem, you might run across this book about how industrial products and software were badly designed, and how to design them better, using new findings from cognitive psychology. But if you went into the bookstore to learn about how to design industrial products or software, you'd end up in the "design" or "software" section, so you wouldn't find it.
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† Right now Amazon's nonfiction bestseller list https://www.amazon.com/Best-Sellers-Kindle-Store-Nonfiction/... has "The Psychopath" at #1, "Mindset" at #2, "Master Your Emotions" at #18, "The 7 Habits" at #21, "Girl, Stop Apologizing" at #28, "13 Things Mentally Strong People Don't Do" at #31, "The Declutter Challenge" at #34, and "Thinking, Fast and Slow" at #50, plus a number of gray-area books; the New York Times has "Think Again" at #11. I think it's fair to say that psychology books sell a buttload.
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† Right now Amazon's nonfiction bestseller list https://www.amazon.com/Best-Sellers-Kindle-Store-Nonfiction/... has "The Psychopath" at #1, "Mindset" at #2, "Master Your Emotions" at #18, "The 7 Habits" at #21, "Girl, Stop Apologizing" at #28, "13 Things Mentally Strong People Don't Do" at #31, "The Declutter Challenge" at #34, and "Thinking, Fast and Slow" at #50, plus a number of gray-area books; the New York Times has "Think Again" at #11. I think it's fair to say that psychology books sell a buttload.