Most likely because there have been very few black authors in the sci-fi/fantasy genre. People who write books tend to write them based on their own life experience and will usually put someone who looks a lot like themselves into the primary roles. Historically most writers come from an upper/middle class Caucasian background. Nobody is out trying to discriminate against blacks in sci-fi/fantasy. You might as well ask why most books from India or China don't have many white protagonists in them.

As an avid reader of those genres myself I couldn't give a damn what "race" any of the characters are from as long as the book is good. I'm very confident that nobody else I know who reads a lot cares either.