The popular idiom is “Don’t judge a book by its cover,” but despite how true it might be in an idealistic sense, the reality of the book industry is that the vast majority of readers do indeed use the cover of a book as a deciding factor in not only whether or not they should buy the book, but if they will enjoy it as well.
With millions of books for readers to choose from, the first “sales pitch” is the cover. If it is not striking enough to draw attention, it will be passed over for something more interesting on either side.