Originally it's because black people were seen less and it felt shitty that light skinned people were cosplaying dark characters. Something America has had trouble with in the past. Although now, I feel like there are more blacks seen more so it's not much of an issue anymore. As long as either party is not doing it with the intent of being offensive or making fun of another race, it's fine.
Personally, I don't mind either way, but I do get annoyed when I see a dark person cosplaying a light person when they could pull off an actual dark character better instead, but that's just me being nitpicky on accuracy.