Think its something to do with gearing....
So you run an 18t freewheel on the rear, you gotta run like 22t upfront, weighs more, bigger sprocket looks ugly, lack of bashrings, catches frame etc.
Then you run 12t sprocket on the rear, 18/16t upfront and theres loads of clearance, more bashrings and cranks available, lighter, better gearing.....
Not sure though.