@Chlidonias: The lack of an aquarium (building) in most zoos in the United States (in opposite to most European Zoos) is a typical thing and goes back to the early days of zoo history in America. And I guess, we can only speculate about the reason(s). Maybe most zoos doesn't want to invest a huge amount of money (building itself and further maintenance). Maybe there was not that much interest from visitors and/or zoo staff. Maybe they avoid (later) to compete with pure aquariums (We have elephants, they have sharks, both can live with that). Or maybe they had (and sometimes still have?) the opinion, that fishes do not belong to a zoo because zoos are for terrestrial (and amphibious) animals only...
And because you mentioned the age of SDZ and Seaworld: Well, there is another marine life facility in San Diego (La Jolla), the Birch Aquarium at Scripps, that oppened in 1903 already...
The development in this case seems to be different. In general I would say, that in the last 2 or 3 decades the trend in Europe (Oceanographic Valencia, Ozeanarium Lisbon, Aquarium Genua) and Asia (e.g. SEA Aquarium Singapore) was to built new Aquarium-only-buildings. In the US, I couldn't figure out a trend for the same time. You have zoos who extended their fish numbers like Zoo Miami with Amazon and beyond, but you have also complete new Aquariums as in Atlanta and Long Beach.
By the way: A good question is why Taronga Zoo Sydney (your part of the world

) did not rebuilt its aquarium.
@uszoo: Don't forget that Geomorph has only listed animals (and animal signs) he has seen at San Diego Zoo. There might be more.
On the other hand there is no doubt that SDZ has reduced its animal collection, specially the mammal species (thanks to Elephant Odyssey).
@zooboy28: Are there no more Keas at SDZ?