I suspect the same reason it's fairly difficult to get high quality sushi fish as a regular consumer, the high end markets buy it all up.

But you also have the problem that OP described where people literally have no idea what quality actually taste like so they can get away with selling mediocre fish. Nearly all fruit in the United States is another example. My cousin by marriage was born abroad and he absolutely refuses to eat his favorite fruit, mango, because so much of it is outright trash and even a "good"mango sucks in comparison