why make a distinction between 55 and 60 degrees based on metallurgy?
Above 1/4" the TPI of BSF is coarser than cycle and a coarse thread works better in a soft material as the thread root is thicker, 1/4" was just an anomaly at 26 TPI. Whitworth would have been a better choice and was used in alloy once they had gone Unified. Nothing to do with 55 vs 60.
was there a real engineering reason or was it just because somebody on the shop floor refused to buy a new set of taps?
No just a rule set on the drawing office for the reasons I set out ref 55 vs 60, BSF in alloy and cycle in steel. Once it was on the drawing the shop floor and buying departments had to follow.