Most textbooks sold are bought by students because they were required for a course. Students are not choosing a textbook by cover because they're not choosing a textbook at all. Professors choosing which textbook to assign are doing so based on the content, because that's what they'll be teaching. Professors also get a lot of free sample copies, and are probably choosing between those instead of purchasing their own set of candidates based on the cover.
Most textbooks sold are bought by students because they were required for a course. Students are not choosing a textbook by cover because they're not choosing a textbook at all. Professors choosing which textbook to assign are doing so based on the content, because that's what they'll be teaching. Professors also get a lot of free sample copies, and are probably choosing between those instead of purchasing their own set of candidates based on the cover.