The term "black" was coined by European colonizers to describe their west African slaves that worked in the crop fields of North America. They had lost their language and culture so they had to unite based on their skin tone, features and struggle. Historically, African nations/peoples have never described themselves as black. They only identified themselves with their ethnicity or tribe.
Plus, you have the negative connotations attributed to black people and some people, unfortunately, do not want anything to do with that.
You also have dark melanated people in South India/Shri Lanka and aboriginals in Australia.
For example, you would probably assume these people below the picture are Bantus but they're actually Melanesian people from Papua New Guinea.
These people are not considered black so why should Cushitic people also identify themselves as black? Skin colour doesn't determine your race. I mean you'll find some of the whitest people in Siberia but they're actually not of European descent. They're actually related to the Mongols and have slanted eyes and high cheekbones yet not considered "white".