They didn't own any slaves. They were too poor to own sheep. In fact, vile unsubstantiated yankee propaganda has it that they were run out of North Carolina for STEALING sheep.
They weren't fighting to keep slaves, they didn't have any.
The war was in great part about slavery. But anyone who says it was ALL about slavery is a victim of liberal education, or a liar.
I don't think anyone has said or believes that every soldier who fought in the Civil War did so because they were fighting for or against slavery. For the most part, soldiers fight for home, family, friends, and native land. They fight because it becomes a question of "them against us." Many soldiers on both sides fought for ideas of freedom, as well.
But these reasons apply to every war, so they don't explain why wars are fought. To figure that out we need to look at the speeches of statesmen, newspapers, books, laws and debates. There were other issues. Events and passions had their own momentum, and people were drawn into war for the union or their section by emotion and reaction to provocations and changing circumstances. But if we want or need to find a root cause, slavery is a good one. Slavery and the conflicts over its defense and expansion were the chief reason for the first secessions, and secession sparked the war.
So, I don't think anyone's said that slavery was the only reason for the war, save perhaps in this sense: the US wouldn't have become a country so divided that intersectional war was possible if it hadn't been for an issue as divisive as slavery. It's hard to realistically think of another issue that could have destroyed the political system to the point where the country would split into two hostile nations facing off against each other. Though even here, one could argue that it wasn't slavery alone that did this and that something like the territorial question and the way it was botched were necessary to bring things to the point of war.
Some say "it wasn't all about slavery" as a prelude to simply brushing slavery aside as an issue, and making the war about something else. A lot of these people aren't so much interested in what happened and why, as in ideas of guilt and innocence, sin and purity, moral equivalence and moral superiority. Such issues can hardly be avoided, but it helps to know if people are arguing about different things while thinking that they are debating each other. So often the real argument isn't about what caused the Civil War, but about "my ancestors were better (or no worse) than your ancestors."