I don't presume to speak for Rikter, but I'm guessing it's because, for whatever reason, CBS/Fox chose to use a black banner on the front of Jedi instead of gray. If the back design is such that it lets you bring a background color around from the front, which is perfectly doable with a simple text and photo-boxes layout, then carrying the background color over helps tie the whole piece together. I also think the gold searchlight logo looks a heck of a lot better on black than on gray. You just have to decide which is more important, for the individual cover to look as good as it can or for the items in the set to match each other.