I feel an aspect of this is that very few people are interested in an individual fish. People enjoy seeing fish in schools, in large tunnels, in colorful reefs, groups of cichlids without knowing what they are. They are more attractive as a sum than individual parts; which is why massive open ocean galleries, large reef exhibits, and shark tunnels are three of the most popular sorts of of aquarium habitats. Even sharks, which people may have some familiarity with and may be able to name a few species, are often held in multispecies tanks. The individual fish means very little in all of this.
My thought is that this is because there is much less interest in fish behavior; it is much more difficult to anthropomorphize fish behavior as you can with mammals, birds and even reptiles -- to the average person, they swim, they swim, sometimes they eat, and they swim some more. While I am sure many zoochatters can describe more complex and interesting behavior, I can think of very little that the average person may be aware of. This contrasts how casual visitors often want to see a lion roar, see a monkey swing. see a kangaroo hop.
The prevalence of such beautiful multispecies exhibits also makes it harder to focus on the unique traits of individual species. Signage that tells you the name of a fish and where it live is often the best you can hope for, in comparison to mammal exhibits that may sometimes have several signs explaining different behaviors or adaptions in detail. When you have two dozen species of fish you cannot dedicate that level of detail to every species contained.
It is unfortunate, I often wish I was better educated about fish.