Oxygen-Propane has numerous advantages over Oxygen-Acetylene
for some aspects of metalwork/welding/cutting/treating.

With real steel fabrication; not the local podunck sticking together trailer hitches and railings, such as structural steel, ship building, heavy equipment stuff; likely most if not all preheating is undertaken using 02- Propane because propane is less expensive and has a significantly higher BTU output. It actually burns differently than Acetylene so it's not as good for certain uses but better for others.