Good question. It’s been a while since I asked someone so forgive me if my memory is a little off, but if I recall the reasoning is that when sushi became big in the Edo era it was eaten by hand because it would fall apart easier. But times change and as techniques got better and you could eat it with chopsticks, that became the more ‘proper’ thing to do. Keep in mind that in izakaya (like a Japanese pub) people will even eat French fries with chopsticks - it’s just the thing to do (again, unless it’s super informal with friends and people are starting to get tipsy and then they may start snacking by hand bc they just can’t be bothered lol).

So I guess the reasoning is that the super high class joints pride themselves on their heritage and the “way it used to be done“ rather than worrying about modern sensibilities.

All that said, the best advice is watch what people around you are doing in a situation and follow their lead. That’ll usually keep you in pretty good graces. :)