perhaps one reason people don't marry up or down often is that they do not tend to frequently interact. In a professional environment how often do you meet up with people not as the same or nearly the same level as you? Do many workers fraternize with their bosses when many are just more likely to complain upper management doesn't understand?

I would put most marry up/down on those who have frequent non professional outside activities. Whether its religious, a particular hobby, or simply a circle of friends whose careers all diverged.