Logistics
The main reason is cost. A plasma-cutting process can produce a high level of carcinogenic dust. This dust must be captured in an extraction filtering system. This and the cost to completely cover the machine to evacuate it raise the price significantly beyond what an average fabricator is willing to pay to get a beam coping solution.
The high quality and speed of plasma cutting heads make them very popular for cutting stainless steel. Plasma heads can't be moved close enough to the flange due to their size. This makes it impossible to use many types of copes normally required in beams.