Another angle to look at intelligence is that not all species need it, or need it to same degree. If you are a cow, or a crocodile, then you are a 1-trick grass-munching or zebra-munching pony, and have no need for intelligence. A generalist species like humans, that lives in a hugely diverse set of environments, with a hugely diverse set of food sources, has evolved intelligence (which in turn supports further generalization) to cope with this variety.
At least to our own perception, and degree of understanding, it would appear that the ocean habitat(s) of dolphins are far less diverse and demanding. Evidentially complex enough to drive their intelligence though, so perhaps we just don't understand the complexity of what they've evolved to do.
Evolution is a blind, dumb optimizer. You can have a mutation that is over-kill and if it doesn't actively impede you in some way, it just stays. It's not like it goes, "Ok we need to reduce this to the point where it's just beneficial enough etc".
That said, i definitely would not say the Ocean is particularly less diverse or demanding.
Even with our limited understanding, there must be adaptations for Pressure, Salinity, light, Energy, Buoyancy, Underwater Current etc that all vary significantly by depth and location.
And the bottlenose dolphin for instance lives in every ocean of the world except the Arctic and the Antarctic oceans.
At least to our own perception, and degree of understanding, it would appear that the ocean habitat(s) of dolphins are far less diverse and demanding. Evidentially complex enough to drive their intelligence though, so perhaps we just don't understand the complexity of what they've evolved to do.