Pretty straightforward for me, there's no evidence that they don't feel pain and it's a pretty hotly debated topic. There likely won't be a way to tell if they do feel pain or if it's just stimuli response.

https://news.nationalgeographic.com/news/2013/13/130208-seaf...

A secondary reason is that it's really easy to confuse people who do choose to eat meat about specific dietary concerns, especially when traveling globally. The Japanese for instance put fish into nearly everything and genuinely have a hard time understanding the difference between a vegetarian/vegan/etc there's also been some confusion between daishi and fish. Add to that the number of people who claim to be vegan/vegetarian but don't adhere to it at all and it can make a lot of people see it as a fad diet versus a diet adhered to rather strictly in other people. Another issue is religions that say fish is not meat. This isn't even getting into products people don't realize comes from meat. By just saying I don't eat anything that can feel pain it makes it significantly less likely someone will accidentally feed me meat. My wife on the other hand eats fish/shellfish. Our daughter chooses her own diet and has so far chosen to be vegetarian after deciding she no longer wanted to eat fish. To each their own, everyone has to be comfortable with their own choices.