For example, why opposing slavery is more appealing than perpetuating it? I am looking for a historical pragmatic answer rather than a "moral" answer. Or why bother uniting? Why more states and their people (if it's democracy at that time) choose or vote to fight for the union/anti slavery/"north" side.
In the U.S. Civil War, the Union ihitially enjoyed a preponderance of states, 22 to 11, over the Confederacy, with a 5 to 2 manpower advantage, and a 9 to 1 advantage in industrial output.
Some of the "Union" states (Maryland, Delaware, Kentucky, and Missouri) were actually "Southern" slave states mostly south of the Mason-Dixon line. And two more states in similar latitudes, "bleeding Kansas" and West Virginia (which seceded from Confederate Virginia) joined the Union after the beginning of 1861, making the tally 24 (Union) to 11 (Confederate).
Why did the Union enjoy such a preponderance of states incluidng the six "border" states. Is there any informed opinion that suggests that one or more of the border states would have made a diifference if they had gone with the South?