It may be the fact that these kilts were always meant to be worn with military tunics of the time, which is the reason for the high rise waist line. The straps were generally placed below the waist because the belt (either P08 web or P14(?) Leather) would be worn OVER both the kilt and tunic at the natural waist. It would be very uncomfortable to wear it there if the straps and buckles were also at the same placement.
Just a thought.
Chad