If different people around the word have different ideas about the size and shape of the earth, we simply say that most of them are mistaken. That doesn't seem to work with morality. It seems like the size and shape of the earth is somehow 'out there' to be discovered. In John's terminology, it is objective. 

But is there anything 'out there' in the case of moral principles? Are there moral facts of a sort that are not based on human choices and attitudes? Are the good and the right somehow built into reality or into the nature of things?

For many people the answer is 'No." So the way is open for people to create many different moralities and none of them can be shown to be the one true morality by checking it against the nature of things. Ethical relativism seems to allow for that. That makes is plausible to a lot of people. It might be nice if we could appeal to science to solve the problem, but it seems incapable of doing the job.