It wasn’t only about slavery, Lincoln wasn’t worried about slavery, it was also about taxes.
At the time most, federal tax revenue as generated by tariffs. There was no personal or corporate income tax.
In 1828 congress had passed a tariff of 62% on almost all imported good. Given the south lacked manufacturing capacity, it relied on imported goods. Now South Carolina declared that tariff and the 1832 tariffs unconstitutional, Jackson didn’t accept that but there was a compromised tariff in 1833. There were more tariffs in 1842 and 1857, which fell more heavily on the south then the north, the north took exception to the Fugitive Slave Act (1850) with some states refusing to enforce that law. In 1861 you had the Morrill Tariff, which once again raised the tariffs.
Now between the tariffs and Fugitive Slave Act, you had a lot of friction between the north and south. Taxes which the south saw as unjust, some states in the north saw slavery as unjust. And Lincoln wanted the union to remain intact, he didn’t really care about slavery.
That lead to the next point secession, can a state secede? Lincoln at one time was in favor of secession
“Any people anywhere, being inclined and having the power, have the right to rise up and shake off the existing government, and form a new one that suits them better. This is a most valuable, a most sacred right — a right which, we hope and believe, is to liberate the world. Nor is this right confined to cases in which the whole people of an existing government may choose to exercise it. Any portion of such people that can may revolutionize, and make their own of so much of the territory as they inhabit.”
http://teachingamericanhistory.org/library/document/the-war-with-mexico-speech-in-the-united-states-house-of-representatives/
Now add in the Tenth Amendment “The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.” To put it simply, if the Constitution doesn’t say the federal government can do something it can’t.
So it’s clear how a state is admitted or formed, but it doesn’t say how it can leave, and given the federal government wasn’t granted to power to say no to secession, it can be argued that the states had (notice had as the supreme court ruled later that they don’t) the right to secede. In fact, it was probably one of the reasons the north didn’t try the southern leaders. Because if the court decided the south had the right to secession, the south could have demanded the north pay damages.
So to say it was about slavery isn’t the whole truth, it is part of the truth, there were many issues, taxes were a problem, the way of life in the north vs the south. Urban vs rural. It wasn’t a cut and dry issues about slavery as Lincoln only wanted the union to remain together, with or without slavery. And notice when Lincoln “freed” the slave he didn’t free them all, Delaware, Maryland, Kentucky and Missouri, it was still legal to have slaves.