In an article examining Samuel Delany's literature, Sandra Y. Govan suggests two possible explanations for the failure to meaningfully include blacks in the futuristic picture. One reason, she writes, is that science fiction frequently suggests that humankind will have to conquer petty problems like racism in order to unite against extraterrestrial invaders and other non-human monsters (44). The second possibility points to publishing industry politics, in which editors and publishers reject stories and novels with black protagonists because they are certain such stories will not appeal to the "(presumably white, male)" audience (Govan 44). Similar explanations might account for the absence of women, but simple sexism can explain the cases when women are present in their traditional subordinate, objectified roles. In addition to institutionalized racism and sexism, naivete and myopia as likely reasons for the oversights, allow me to add a profound lack of imagination.
These "conservative" visions of the future overlook the diversity and creativity of humanity, particularly in the areas of "humane" sciences like politics and psychology, fields in which science fiction could be wonderfully subversive rather than a simple mirror parading as a looking glass (Broderick 55).
Broderick, Damien. Reading by Starlight: Postmodern Science Fiction. London: Routledge, 1995.
Govan, Sandra Y. "The Insistent Presence of Black Folk in the Novels of Samuel R. Delany." Black American Literature Forum 18:2. Terre Haute: Indiana State University, 1984. 43-48.