The short answer: Wikipedia editors are volunteers and African-Americans rarely volunteer.
The medium answer: African-American editors often edit only articles which relate to African-American and do that in a point of view way.
The long answer: large blocks of African-American are oppressed, unemployed, poorly educated, and computer illiterate. Those that are educated and prosperous tend to be too busy, and as said, are not in the habit of volunteering.
Another matter, although lip service is paid, few African-Americans have an interest in Africa, at least not enough to read and edit Wikipedia.
African-Americans who live in ghettos in the inner city do bear some resemblance to Roma, the educated not so much; they are generally not entreprenurial as Roma are; they tend to take salaried jobs.
All that said, we need to be as welcoming as possible, create good Wikipedia editing projects for them to plug into, and reach out when the opportunity arises.
Fred Bauder
...Wow. Maybe you can follow Phoebe's example and cite some evidence?