they were designed for commercial use and being able to hide everything around your hips while you're working up by your head. It is also very difficult to reach behind your head in the types of suits they had to wear while diving and if they mounted the tanks high enough to where they could do that, they'd be so top heavy they would fall over, so it helped with balance since they were most often vertical in the water. It's a moot point for most modern divers that use them since we are able to easily manipulate the valves and are normally in a horizontal position. Placing the valves at your hips while you are horizontal will put you feet down, which is bad