Why was Abraham Lincoln's concern more focused on preserving the Union, rather than slavery?

The main plight of Abraham Lincoln was preserving the union because it was the moral and upright thing to do. As well as the united states was fairly new, separating like this is a sign of weakness one could say. Also Lincoln did use this to support his apposition towards slavery by issuing the Emancipation Proclamation in January of 1863, which freed the slaves in the confederate states, ironically he had no right to do so since they declared themselves a separate nation. So in a way he combined his quest for persevering the Union and slavery together with the "persevering" part in front so in a way its like killing two birds with one stone.