LKW; I would posit that there were probably several root causes. I'll suggest a few;
-> Need for small frames with upright captain's position
-> Need for easy fitting of those couplers to yield take apart frames
-> Maybe someone did the math and found it works well enough
-> Maybe it saves 100 grams
-> Maybe to ease the mitering on the tubes or the number of angle settings
-> Just to be different?
Haven't seen one yet that had much appeal for what that matters.
/k