I’m not. In Collapse, Jared Diamond wrote that cultural food taboos involving meat, fish, or shellfish are common all over the world (of course, not all of these taboos involve the same species). He theorized that this might be because meat, fish, and shellfish are more likely than plants to develop bacteria that can lead to food poisoning, or to harbor parasites that can infest humans.
Learned food aversions are common in people. That’s when you get sick after eating some food, and can’t stand the taste or smell of that food afterward. This is found in a lot of other animals as well, and probably evolved as a strategy for avoiding poisons. If fish and seafood are more prone to harbor unfriendly bacteria than other foods are, then it shouldn’t be surprising if there are a lot of people out there with aversions to fish or seafood.
There’s also the fact that shellfish and fish allergy is common. Shellfish, fish, dairy, soy, wheat, peanuts, tree nuts, and eggs are the eight most common food allergens in the US. Those are the foods that you’re likely to see “contains X” or “processed on equipment shared with Y” warnings about on packaged foods. Food allergy symptoms can include nausea or vomiting, which could trigger a food aversion. Some of the people who don’t eat shellfish because of an aversion to it might develop a shellfish allergy, if they were to eat shellfish.
Shellfish can go bad much faster than chicken or beef. Fish has the problem of going bad quickly, as well (I keep kosher, so I don’t have much experience with shellfish). I’ll buy chicken or beef a week or so ahead of when I want to use it. Fish, unless it’s frozen, I prefer to buy the day I’m going to use it, or the day before at the absolute maximum. I’ve found this is the key to avoiding that fishy taste- don’t keep the fish around in the fridge for a long time, and never buy any fish from a fish counter that smells fishy. I’ve found I need to be much pickier about where I buy my fish than I do about where I buy chicken or beef.
Shellfish also tends to be costly, per pound. If something is expensive, a cook is less likely to throw it out if it’s a little past its best. It’s much more tempting to eat something that has been around a few days and maybe smells a little off if it cost $20/pound than if it cost $5/pound.
A mild allergic reaction or eating some less-than-fresh seafood could easily trigger a food aversion.
Those of us who like fish shouldn’t be trying to push them on others. We’re having enough trouble harvesting enough fish for the people who like it now to have it. There really is a limited supply of fish in the world. Don’t waste it on people who don’t like it. That makes it more expensive and harder to get for us.