About 15% of the American population is black. The majority of the population is white. Therefore it seems logical that most authors are white, and by extension the fictional characters they write about. It's no mystery. America has a free market i.e. no restrictions on what types of litterature that can be produced.

A shortage of black characters is just a natural consequence of being part of a minority group. It's not an injustice, since the market is supposed to represent the people, and the majority population is white. Besides, as far as I know there are already stories with black characters. Those stories just don't dominate the market, for some reason. Maybe because they tend to revolve around race, and that's just not that interesting of a topic.