There are two reasons. First, there is distributivism. The Federal government has no business in enforcing its will on states in contravention of the Tenth Amendment. Second, I do not agree that there is such a thing as an hereditary contract; if slavery is morally insupportable, then so is Lincoln's idea of an indissoluble Union. They are both based on the concept that it is possible to inherit a condition of servitude -- in the one instance to a single master, and in the other instance to an institution, the Federal government. Whether or not the southern states seceded because of slavery or tariffs is immaterial. In my opinion, they could have seceded because they didn't like Lincoln's hairstyle. In effect, the southerners were simply refusing to renew a contract that they never initiated, unless one assumes the legal fiction that the persons who ratified the Constitution in 1787 were "virtual" representatives of ALL who came after - a sort of political version of Original Sin.
The gradual assumption by the Federal government of absolute power and authority follows a set pattern:
1) Locate an "evil."
2) Drum up popular support to eradicate that evil
3) Re-interpret (or totally ignore) the Constitution, in order to
3) Pass laws to eradicate the evil . . .
. . . and here's where the fun begins . . .
4) Construe the laws passed in 3) as the new de facto interpretation of the Constitution, thus bypassing the amendment process.
5) Use this precedent to extend Federal authority into areas not directly related to the original evil.
6) Repeat as desired, until the Constitution comes to mean the opposite of what is written and what had been (under "original intent") commonly understood.
This was done with slavery, racial discrimination, drunken driving, the war on drugs, child pornography, and terrorism. Each evil has been used by the government as a lever to pry apart the Bill of Rights, by playing upon the good intentions of an easily deluded, but well-intentioned, electorate.