It's not quite that simple. Every new state admitted came to the party with two new Senators -- so each new free state had the potential to shift the balance toward abolition. So the question wasn't just the expansion of slavery on the continent, but the expansion of abolitionism in the Senate.
All the compromises of the early 19th century were, from the Southern side, about maintaining the existing balance. It was a defensive posture.
There were 15 slave states in 1861. Had they all held together then in order to adopt an amendment ending slavery 46 states would have to ratify it. Do the math.
The Republicans knew that there was no way they could end slavery in the face of opposition from the slave states. But they did believe that Chief Justice Taney was wrong in Dred Scott and that the government could keep it from the territories.