They may have been made scarce by many methods, perhaps a low print run; court rulings; burning in a revolution to suppress dissenting views; scanning to avoid competitors to get the content and prevent other LLMs to reference, search, or train on it. Perhaps it's a particular edition that is rare (e.g., the first edition had a different description and was thoroughly changed in the second edition of the book, which may be important to have all the facts from someone's biography).