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

Abraham Lincoln was more concerned with preserving the Union because it mattered more for the future of America. If the Union broke apart then everything would go down hill, like how the country would turn out, trade, and having possibly another enemy. Slavery was also not important in helping a new country become more powerful and therefore would not matter if it fell apart. I also think that Lincoln did not want to seem responsible for the Union falling apart under his presidency. It would make him look bad and he would also be held responsible.