That it took Lincoln a long time to emancipate is of little concern strategically. His stated goal was not emancipation but reunification. He chose whatever policy he thought would work best toward that goal. Large numbers of Union soldiers hated the EP and large numbers strongly opposed the idea of black troops. In very little time however, they had a dramatic and sudden, and almost universal change of heart and wanted more black soldiers sharing combat responsibility. They quickly realized that the more blacks in the fight lifted their burden and their risk. The white Union troops realized more armed blacks in combat improved their chances of seeing home again.
As this became more obvious to everyone the practice became more popular to troops and family alike and was increased. Lincoln adjusted his policies as they furthered his main goal. This does not seem to be the case with the CSA leadership for the most part. Lincoln adapted they did not. I personally think his administrations' policy of stopping POW exchanges was brutal and inhumane to loyal Union men suffering in captivity.. but it was effective policy as it denied the South a return of combat soldiers.
It was a horrific sacrifce, and a very very hard choice to make. The CSA had a hard choice to make about arming and freeing slaves and even in the end could not bear to do so.
But in the South, they would forcibly draft almost any and every available and able white male of draftable age, and later included boys and old men, and a family might give every single one of their white sons to the cause, while the black half of the population mostly stayed put. Safely out of harms way.
Even when arming slaves became policy in February 65, only 20% of a regions slaves could be taken. Priorities. We will risk every last one of your white sons, who are expendible, but only a fifth of your black slaves, who we cannot risk losing.
This doesn't even begin to consider the additional cost of the white manpower kept away from active combat because they were needed in order to man local patrols to control potential runaways slaves or uprisings, and keep a check on the slave population, and keep them working productively. Maintaining slavery was costly in white manpower to a government that clearly needed more men, yet was unwilling to adapt.
If you were a father and had to send one member of your household to face grave danger, maybe never to see him again in this life, which would you chose to throw into harm's way... your only son or one of your field hands? In a sinking lifeboat which would you toss first, to lighten the load, and maybe save your life, the anchor, or your trunk of valuables? Priorities are revealing.