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

Abraham Lincoln's concern did not focus on slavery because he knew if he had made any opinion on slavery he'd either lose many of his followers, or be unable to convince the South to revoke their succession.
If say Lincoln stood up and mentioned his hatred for slavery, the South would have more reason to succeed because they were already succeeding on the grounds of their rights being taken away, why come back when the man with whom is running the North government is planning to revoke more of your rights?
Now the other side, if Lincoln had come to a podium and mentioned his love for slavery, he may have gained some relations with the South, he may lose many of his own Northern followers as well as the chance he may be impeached for this belief.
That is why Lincoln concerned the preservation of the Union rather than the conflict of slavery.