There were at least three main considerations that led Lincoln to issue the Emancipation Proclamation.
First, there was his personal opposition to slavery. Though Lincoln was not an abolitionist, he did believe that slavery was evil. Therefore, he was not sad to find a way to get rid of it. However, Lincoln’s first goal was to save the Union. This means that the main considerations that led to the proclamation had to do with helping the Union to win the war.
One way that Lincoln hoped this would help was by inspiring slaves in the South to rebel or to leave their masters. Lincoln knew that the South relied on their slaves and that the loss of those slaves would gravely reduce their economic capacity and their ability to fight.
Finally, Lincoln hoped to keep European countries out of the war. If Lincoln issued the proclamation, the war would come to be about slavery, not simply about whether the South could secede. England and France would be much less likely to help the Confederacy if the war were about slavery rather than about political goals.
For these reasons, Lincoln issued the Emancipation Proclamation on January 1, 1863.