The cover does not matter for a textbook.

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.