Running a zoo is not a profitable business, this is why there aren't a ton of private zoos. Keeping that in mind, they do the best with what they have. I'm guessing that when they have a little extra money laying around it's not going to go into extra maintenance for the tiny saltwater section that they have since that is not what most people go to the zoo for.
Try to look at this in a positive way, at least they do have a saltwater section that kids can see and be excited about. They will be looking at the brightly colored fish and the corals they do have. 99% of the population isn't going to know what an aptasia is let alone focus on it or bemoan the lack of coral variety.
If you feel so strongly about the condition of their aquarium, try contacting the zoo and see if you can volunteer to help maintain it.