Mainstream America will support ANYTHING that's popular; it doesn't matter what it is. As long as it's profitable, it will be promoted and marketed to bring in more money. What do they have to lose by there being black sci-fi films?
Most of us here fit into the geek/nerd category, but we are a minority amongst other blacks. Not only are we a minority, but we are also often ridiculed for it..not so much now as even 10 years ago, but it's still frowned upon. it starts there.
High concept, well done Sci-Fi/Fantasy flicks are expensive as hell, and these studios want guaranteed returns in order for them to even greenlight them. We are talking about budgets entering the triple digit millions mark.
The studios are focused are making the maximum amount of profit. They believe in order to do this, they must focus on drawing in the "general audience." (White people and people who are only comfortable with seeing white people on the screen. Or if you're Asian, you pretty much only want to see White people or Asians on the screen, and no one else. This is particularly true of the Chinese.)
Movie studios think that the only black men who can draw enough of the "general audience" to a movie as a lead are Will Smith and Denzel Washington, and to a much,
much lesser extent Samuel L and Morgan Freeman (the latter two aren't likely to be leads as much as prominent cast members). There is also a general sentiment that black leads can't draw overseas, especially in their precious Chinese market. Hell, just look at how one executive viewed Denzel's draw potential overseas in the Sony leak:
Sony hack targets Denzel Washington and 'racist' overseas audiences
As I stated above, he and Will are considered the two black leads they can depend on as universal draws, and yet look at how he is viewed as an overseas draw!
They refuse to take a chance on black leads and black casts in general outside the established "race roles" because they want to make money. And as far as they are concerned a white lead male or female is guaranteed money with "general audiences", and a black lead is the exact opposite.
Dwayne McDuffie discusses the "Rule of Three": Where if three or more black characters are in a comic book series, it's considered a "black product" and thus many white readers, whom are the overwhelming majority of comic book buyers, ignore it. The thing is, this doesn't just extend to comic books, it extends to all media. And replace "white readers" with "non-black viewers."