It isn’t; someone might perfectly well hold that ethical sentences express both propositions and emotional attitudes. But those people would not be classified as emotivists. It happens that some people hold the more specific position called emotivism, and it’s useful to have a word for it.
Emotivism is a meta-ethical view that claims that ethical sentences do not express propositions but emotional attitudes. Why is it an either or?
It isn’t; someone might perfectly well hold that ethical sentences express both propositions and emotional attitudes. But those people would not be classified as emotivists. It happens that some people hold the more specific position called emotivism, and it’s useful to have a word for it.
Because most people cannot count any higher than one.