Maybe because they loved their product and were protected by the government via tariffs to sell their wares at artificially higher prices.
Northern newspapers that were associated with the Republican Party openly advocated protectionist tariffs as a tool of plunder directed at the Southern states. As the Daily Chicago Times editorialized on December 10, 1860:
Cognizant that the Confederate Congress was about to adopt a much lower tariff rate, the Chicago paper warned that if the North were to "let the South adopt the free-trade system," the North's "commerce must be reduced to less than half what it is now . . . leading to very general bankruptcy and ruin."The South has furnished near three-fourths of the entire exports of the country. Last year she furnished seventy-two percent of the whole . . . We have a tariff [the Morrill Tariff] that protects our manufacturers from thirty to fifty percent, and enables us to consume large quantities of Southern cotton, and to compete in our whole home market with the skilled labor of Europe. This operates to compel the South to pay an indirect bounty to our skilled labor, of millions annually.
On March 12, 1861, a week after Lincoln's inauguration and a month before Fort Sumter, the New York Evening Post, another Republican Party mouthpiece, advocated a preemptive strike against the Southern free traders with a naval attack that would "abolish all ports of entry" into the Southern states.
The Newark Daily Advertiser, meanwhile, expressed its disgust that Southerners had apparently "taken to their bosoms the liberal and popular doctrine of free trade," and that they "may be willing to go . . . toward free trade with the European powers." "The chief instigator of the present troubles—South Carolina—have all along for years been preparing the way for the adoption of free trade," and must therefore be stopped "by the closing of the ports" by military force.
When Lincoln was inaugurated his party had just doubled the average tariff rate and was planning on increasing it even more. Then, in his First Inaugural Address, he promised a federal invasion of any state that did not collect the higher tariffs, as South Carolina had refused to do when it nullified the "Tariff of Abominations" in 1832.
As he said: "The power confided in me will be used to hold, occupy, and possess the property, and places belonging to the government, and to collect the duties and imposts; but beyond what may be necessary for these objects, there will be no invasion—no using force against, or among the people anywhere" (emphasis added).
Collect the higher tariff rate, he said, and there will be no invasion. Fail to collect it, and there will be an invasion.
https://mises.org/library/gods-generals-and-tariffs