The South realized it was being isolated as a sort of agricultural vassal state to the Noth.
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How in the world did the South get the Fugitive Slave Act passed?
Because the North (at least the majority) didn’t care about slavery either- certainly not enough to go to war over it.
Let’s look at what the North did fight against - viciously. They absolutely fought against the expansion of slavery into the western states as much as they could get away with. Great, but why fight hard on that but not the other issues? Because it would act as a voting block against Northern interests when it came to other economic issues.
Finally, I think many were also concerned with what Lincoln would do. Lincoln was not above pulling some nasty political tricks to get his way. Instead of waiting and seeing, they acted rashly and seceded without adequate preparation. But don’t judge them too harshly. They and theirs paid a very high price for their mistake. And they could have turned to guerrilla warfare at the end, but didn’t.