So the question remains why?
I recall the altimeter we had in the back in the Varsity. You could set it to zero on the ground or to the airfield elevation - there was no subscale. Once above transition we could set it to SPS.
It would have been possible to reset to QFE or QNH either by winding the height needles by a given amount (fraught with danger) or setting to the same as the pilot's altimeter. In fact neither method was taught. In those days navigation finished at the initial approach fix or overhead.
Now a good reason for using QFE could have been the approach aids of the day - an ACR7 approach with a 300 foot per mile descent. Or a Eureka homing. They were complicated enough at the time so the attraction of a 300 foot per mile system was clear.
With modern systems with precision height systems then QNH has advantages with deconfliction with the ground or other aircraft below transition.