Lincoln, the Illinois Butcher™, freed the slaves when he ran out of Irishmen to send to the slaughter. Out of necessity, he emancipated a fresh new batch of meat, a little darker but more temperate.
I suppose that's the same reason why Davis finally agreed to allow blacks into the ranks in 1865? Having slaughtered almost all of that part of his available white population that hadn't already deserted he was getting kind of desperate?