The game designer explains

(Not very) sorry for the clickbait, but you'll never guess who's most likely to complain about being assigned a black avatar.

The designer says it's a survival game where it's difficult but important to form coalitions, and the assigned visual traits are a way of preventing anonymity for avatars-- you can't betray people and come back looking different. That's looks like an excuse-- avatars could just have permanent names or tattoos or somesuch.

Still, this is a really interesting experiment, and the game is apparently good enough enough that people have been playing it for years.