Why do americans consider the word "negrо" as offensive?

There are negative connotations associated with that word in North America. Most people don't use it in casual conversations. It certainly isn't used the same way as it might be in Latin America - my bf's nickname is negrito (he's about 1/4 afro-cuban, but quite dark), but in Cuba that's acceptable. Referring to anyone as '*****/negra' there is fine since the translation from spanish literally means black, but here that word along with the other N word are associated with slavery/racism/etc etc and looked down upon.