Shoot me down in flames, but...... I have a couple of theories without having the actual facts at my fingertips.
All regular RAF pilots used to be officers before WW2 (unless training when they were officer cadets), but the rapid expansion of the RAF combined with the attrition rate of pilots during the war meant that NCOs were cheaper and quicker to produce. Once WW2 over there was an experiment with new RAF aircrew ranks for NCOs but this was not successful and ranks reverted - it is possible that the demise of the NCO pilot was around the time that this happened (1950).
I believe that during WW2, nations such as RAAF trained their bomber pilots while NCOs, but then gave them a commission before starting Ops tour.
As an afterthought, it is also possible that many NCO pilots were originally RAF VR and not regular pilots, but I stand to be corrected.