Most black people are not into sci-fi you guys can try to deny all you want but it's true.
Just look at books, there are black sci-fi novels but they hardly get any attention because black folks aren't reading them.
The black books that do sell are romance, humor and black struggle type stuff.
And I have a feeling a sci-fi show with an all black cast would fail in a second.
Did you see the Matrix, only white people in it are Agent Smith, Neo, and Trinity