We're always happy to expand on things, but we get questions like this one a lot so we also have a section of our FAQ devoted to just this question. The short version? That's just not so at all. It's slavery.

The slightly longer version is that the other causes of the Confederacy listed, either as primary or in competition with slavery for the top spot, are historically dubious and/or aliases for slavery. In the first camp you'll find unspecified reference to Lincoln or northern tyranny. The seven most enslaved states didn't even wait for the guy to take office and the history of the antebellum United States is not one of the North dictating to the South and forcing the section's submission. It's almost completely the opposite, with the exceptions usually being fig leaf provisions passed by Southern-dominated majorities to help Northern politicians defend themselves on charges of knuckling under. And the North and South don't make any sense as interpretative categories, then or now, unless the sole distinction between them is the extent of their embrace of slavery. What actually looked likely to decrease after 1860, probably permanently, was the South's ability to dictate to the nation through a de facto veto power over national administration.

In the aliases for slavery camp, you have things like "honor" (oddly, the honor just of the slave states), or southern "way of life". The central institution of southern life was slavery. The tariff is a little more complicated, but ends up in the same place. It is a real nineteenth century political issue which has adherents for and against in both sections, so it might look on the surface like a separate thing. That's technically true, but misleading. When the South's fighting a high tariff, it's fighting explicitly to maintain the profits of its slave-driven agriculture. Ignoring that is as far off the mark as arguing that New England farmers might be anti-tariff because they love slavery and want to see it prosper. (Midwestern farmers are a different story on that front, since a lot of their product is shipped south to feed the cotton states.)

The tariff poses further challenges as something for the South to fight against on at least three grounds. The first is that every tariff the nation passed before 1860, it passed with Southern votes. Even the Tariff of Abominations, the highest the rates ever went, had a southern constituency and could not have become law without Southerners signing on. They had parity with the North in the Senate, one of the sources of their de facto veto on national policy. The North did not have this same power, because the South was a far more politically unified section on the matter of paramount concern: slavery. So Southerners are going to unite and rebel against tariff rates that they set?

Second, those rates had gone down a lot from the days of Andrew Jackson. If the tariff were truly intolerable to the section, adequate to justify breaking the nation and hazarding a war, then we would not expect that time to come in an era of low duties. If you care most about getting the rates down then the most radical measure to do so hardly seems warranted when things have gone your way for more than a decade. That might be credible still in a time where the bottom had fallen out of southern profits and so any tariff was too much tariff, but the 1850s are the biggest, boomiest years for southern cash crops ever. The depression of 1857 didn't even touch the South much. Cotton is doing so well that the price of slaves decouples from its price and keeps rising higher, to the point where white southerners worry that slaveholding will become obviously inaccessible to poorer whites and break racial solidarity along class lines.

Third, you have to look a long time to find southerners in 1860 who want to use the tariff as their rallying cry. They are very clear, as documented in our FAQ, that slavery is the big deal. A school of thought holds that slavery (or even white supremacy) was just a political con game by the rich elites to sucker the poor farmers into a bloody war they wanted for nefarious reasons (or just hopelessly abstract) reasons (the tariff being a big one) but southerners are just as explicit about the threat to slavery being their motivating cause when they're speaking legislature-to-legislature and trying to convince fellow elites in states that have yet to rebel that they ought to come along.

But having said that, I should also say that none of this means white Southerners are incapable of thinking about any issue other than slavery. As long as it's under no threat, they have a vigorous, competitive two-party system that contends and splits on national issues at least from about the administration of Andrew Jackson forward. (Before that, saying anywhere in the US has a two-party system is dubious.) They're not robots programmed only to think about slavery, but it is unquestionably the single most important issue for almost the whole section, almost all the time, from about 1800 onward. A credible threat to slavery (or something your political opponents can spin as one) is the section's red line and trumps all other concerns. They may express occasional qualms, sometimes with great eloquence, about the institution's morality or its influence upon white character, and speculate about some future generation ending it, but as a matter of practical politics they are for slavery forever.

That stand can prevail against their attachment to white republicanism, though by the late antebellum that style of rhetoric is long out of fashion except in a few crustier parts and has yielded to white race egalitarianism. Both sometimes make them sound a little bit like misplaced communists bragging about their classless system and castigating northern capitalists. From a certain POV, it's the oldest thing about the American South. Their commitment to the preservation of slavery is even in the Declaration of Independence. The crimes of George III include this:

He has excited domestic insurrection among us, and has endeavored to bring on the inhabitants of our frontiers the merciless Indian savages, whose known rule of warfare is an undistinguished destruction of all ages, sexes, and conditions.

Excepting the authors of that document and their movement, there are no clear populations of would-be insurrectionists in the thirteen colonies in 1776, except the slaves. One could point to the Native Americans, but Jefferson knew his way around a pen. He writes "and" there, not "by" or something to that effect. The two are linked in threatening the physical safety and security of the white colonists, but they are different threats distinct in his mind.

/u/georgy_k_zhukov has a good run-down of the reasons behind the American Civil War here. He shows convincingly that, while things like 'states rights' and tariffs were concerns in the American South in the run-up to the war, these issues ultimately came down to slavery.