Why?
Also...duh. Like it or not, white/lightskinned characters are WAYYYYYYYYYYYYYYYYYYYYYYY more marketable and appeal to the main public of mangas who also happen to be the target ones.
As for usopp, i´m pretty sure most black/brown people wouldn´t like to be represented by such a trash weak coward character as usopp, at least blackbeard exists.
My favorite black characters from anime are dutch from black lagoon and tousen from bleach, anyone else is irrelevant as far as i know.
Also, african americans never dominated culture, they played a role but were never the main show.