Lincoln was under a great deal of pressure from abolitionists, once the Civil War started, to formally declare an end to slavery. Since there was no way to make the southern states any angrier, as they had already seceded and gone to war, abolitionists argued that Lincoln had nothing to lose and everything to gain by issuing the Emancipation Proclamation in 1862.
But it wasn't that simple. Four border states, Missouri, Maryland, Kentucky and Delaware still had legal slavery and were still in the Union, but issuing the EP might well drive them to secede as well, and with the nation's capital wedged in between Maryland and Virginia, it was in a precarious position. Nor was the war going well for the Union.
So Lincoln had to walk a tightrope. He waited until after a Union "victory" (actually, Antietam was a tie) so it didn't seem too desperate, then in his proclamation freed only the slaves in seceding states. Since the Union did not control any southern states, it didn't free a single slave in reality, but it made the abolitionists happy, kept the border states in the Union, and made it more difficult for abolitionist Great Britain to back the South.