For the last year and a half, the U.S. Department of Justice has been trying to explain why nearly everyone convicted in California's federal courts of ''crack'' cocaine trafficking is black. Critics, who include some federal court judges, say it looks like the Justice Department is targeting crack dealers by race, which would be a violation of the U.S. Constitution. Federal prosecutors, however, say there's a simple, if unpleasant, reason for the lopsided statistics: Most crack dealers are black. ''Socio-economic factors led certain ethnic and racial groups to be particularly involved with the distribution of certain drugs,'' the Justice Department argued in a case in Los Angeles last year, ''and blacks were particularly involved in the Los Angeles-area crack trade.''