I still think the question is far too complicated to answer without an awful lot of thought and research, but I decided to stick my neck out anyway and offer some opinions here. Some reasons I can't even fathom. Environment is about animals because they make easy and popular news from a journalistic perspective. Environment is about animals because we perceive them as the indicators of environmental quality that they are. Environment is about animals because they're easier to like than many people.
Oceans are an economic issue cloaked in environmental concern.
Nobody is going to hug a fish. Whales are another story. Most of these topics are complicated and not very huggable. This means that the topics aren't easily packaged for public consumption. No one knows which topics are more important than others.