Since the discovery of sulfur sensitization hinged on discovery that a thiourea was causing it, there was a natural progression in use of heat degradable sulfur compounds. That is my only guess here. IDK why. I do know that formulas from the 40s and 50s used Thiocyanates and then moved to Hypo. Early ones used Thioureas.

I do know (I think, in the back of my memory) that the Thiourea sensitizations were hard to control, and that there was something specific about sulfur + gold where hypo was used. That is it, just a hint of memory.