The Dukedoms of Fife, Northumberland, Manchester, Lennox and Buccleuch have no marquessate, but the rest do, you're right. Maybe it's just a title that's gone out of fashion these days - I mean, since Edward is a prince, it makes no practical difference whether he's a duke, a marquess or an earl, since as a prince and HRH he outranks all of the above (except Andrew!).