I would wager the reason is most people buy cases for their phones, especially if they have killed one just by dropping it. I think the black and white options for the front for personal preference -- I love white on my iPad -- but my iDevices are covered like a knight after I killed my iPhone thanks to a shoddy case.
I'm willing to be many fewer people put any protective device on iPods. I owned a classic and two shuffles and never put a thing on them.