Neat question, clarky, though neither Emily nor I are marine biologists. But I’ll try to answer, and I bet Emily will have some cool answers too.
Three things that happen as you go deeper in the ocean are that the pressure increases, the temperature decreases, and you no longer have access to sunlight. These are all things unfriendly to survival! However, I’m not sure that there are fewer creatures down there. There are lots of species that have evolved to live in absolutely crazy environments. So it probably has more to do with our ability to detect the creatures than that they aren’t there!