Captive bred fish are held in high density for substantial amounts of their lives and fish that die from disease are kept in close contact with healthy fish for some amount of time before being removed.Wild caught fish intermingle with dozens or more different fish at every step of the supply chain. I’m just assuming that would lead to more disease.
Keep in mind that the legislation was written before captive breeding of marine ornamental fish was really a thing and when you look at stocking densities of many aquacultured food fish, they are simply a breeding ground for disease. Even farming fish in sea pens has shown very high rates of disease despite the fact that the pens are continually flushed with new sea water.