Because most of them don’t have a long-term interest in the fisheries. It’s the same reason that employees shouldn’t run big firms. Their goal is to maximize their lifetime income, not provide for the long-term health of fisheries. If you have a demographic situation like the baby boom, you could end up with a big majority of fishermen born in the same decade, all of whom would then vote to maximize the yield of fish over their expected working life. That is, today, a bunch of 30 year olds could vote to manage the fisheries in a way that maximizes their yield over another 30-40 years, beyond which they may or may not collapse. These fishermen wouldn’t care.