I'm pretty sure it was originally because they used BMX rear hubs, so if you had a screw-on rear sprocket and a front freewheel the gap between the chain and the stays was smaller, so if you landed on it you had less chance of snapping it. EDIT: It actually lighter overall to have a rear freehub, or it was when I last added it up, which was before the Try-All H hubs came out.