It may appear that all good gospel singers are African-American because gospel music has long been a part of African-American culture. More African-Americans listen to gospel music compared to other races in the United States, so young African-American children are exposed to this type of music and may be inspired to pursue gospel singing more than children of other races.
As is the case with other art forms, as the fan base becomes more broad and diverse so do the artists. As gospel music becomes more popular, it would be expected that the race of gospel singers will become more diverse.