Sometimes it is because I have a short stock on it and I need to keep the whole thing above the legal OAL for a rifle (26"). This really only comes into play with my single shots.
The other main reason is that when it comes to firearms, I am a bit of a non-conformist. I still love to hunt with single shot handguns. I have quite a few rifles too, but a lot of them are chambered for wildcats and odd-balls. The ones that aren't have some type of weird characteristic, most of the time it is a barrel that is 17.5" or 19.25" or 21". Just a personal thing.