The more primitive body plan of the fish does not have internal lungs to support. Amphibians which are half-fway between aquatic and terrestrial animals have 3-chambered heart, but once the Oxygen exchange is entirely depenedent upon internal lungs an additional chamber and dedicated circuit is required.
-
Fish require less oxygen in their bloodstream to survive than your typical mammal, also they don't have the long neck which needs a much more efficient pump to force the blood to circulate through it.
-
A fish doesn't have to pump blood thirty feet up to its head
-
Smaller body, less complex network of veins.