San Diego already has Sea World, the Birch Aquarium, and a Sea Life in nearby Carlsbad. An aquarium probably is not a priority at the zoo with all of those other facilities near by. The new penguin and shark facility is an aquarium of sorts.
If talking about number of species, yeah it’s insane how many species of plants and animals they can fit in there. It’s actually not a super huge zoo in terms of acreage though, it’s not huge. It sits on 99 acres of land. For reference, LA zoo is 133 acres in size. San Diego Zoo looks so much bigger when visiting. Maybe it’s because of the varying terrain or placements of the exhibits.