I don't know, The Mentalist is an ongoing show, and has loads of longfic (alas, it's mostly ff.net quality, and about the main het ship, which I don't care for).

My guess is it has to do with the nature of the fandom's popularity - you have to be quite committed to a fandom to write longfic, so fandoms/pairings with more passionate fanbases would be more likely to produce longfic in large numbers, whereas a fandom with a lot of casual fans would be more short-fic focused.