If slavery was expanded to the territories, then slave states would continue to have parity, or maybe even outnumber, free states.
This is from the first posting in this thread:
Our greatest contemporary historian of the Civil War, James McPherson, has noted that Confederate President Jefferson Davis, a major slaveholder, “ justified secession in 1861 as an act of selfdefense against the incoming Lincoln administration.” Abraham Lincoln’s policy of excluding slavery from the territories, Davis said, would make “property in slaves so insecure as to be comparatively worthless ... thereby annihilating in effect property worth thousands of millions of dollars.”
If the number of free states continued to expand, and the number of slave states did not expand, the free states would outnumber slave states and the free states could eventually vote to change the Constitution to explicitly outlaw slavery. Southerners who had a lot of their wealth tied up in slaves would lose much of their wealth. Even Lincoln Republican statements that slavery was wrong and should not be extended to the territories was threatening to slaveholders, because if it became universally accepted that slavery was wrong, then it would be expected that slavery should end.