Good question.  The reason is that I/we don’t get to game super regularly so when we do game we try and pick a system and run it for a while. Which often means one or two sessions a month if we are lucky but usually it’s one a month then maybe off a month or two between sessions.  Flipping systems gets Cumbersome as you are constantly trying to work up momentum over that kind of time frame.  

When we do get to gaming even in a trad game there is that moment of getting everyone together and back in the jive.  It’s always easy to slay the Orc, it takes a while (at least for us) to get back into the character drama.