Reliability
Another issue is that plasma heads can be damaged if the flame blows back when cutting thicker sections of material, even with wide-flange beams.
The most important factor is, of course, cost. Plasma cutting produces extremely carcinogenic dust, which must be captured in a dust extraction filtering device. The cost of enclosing the machine completely to evacuate the dust increases the cost far beyond what a typical fabricator would be willing to pay for a beam-coping solution.