Why did African Americans choose nonviolence?

During the civil rights movement (1955-1968), why didn't African Americans choose to be more militant against their oppressors? They had so many grievances - slavery, jim crow, lynching - for so many years. I would imagine that they would have been more aggressive in their resistance. Why did nonviolence become popular specifically during this era? I know it was developed under MLK but what about that time period made nonviolence an effective means of protest?