As someone who spends a fair amount of time working on print publications, a preponderance of white covers probably represents the designer just taking the path of least resistance. The minute you move away from white backgrounds, you start dealing with issues of color harmony between background, print font and product and a colored or grey background can make elements in the visual composition either look muddy or "pop" too much. Also, different cultures have different emotional associations with varying colors and if you are trying to appeal to an international audience, white is a safe bet.