1) No war ever has just one cause. No individual is ever just motivated by one point, for that matter. Human psychology is complicated.
2) In any era, 55% is landslide public support. Even assuming a good democracy, even assuming the assumption that the South was fighting to free the slaves, this does not mean every or even the vast majority of Southerners were pro-slavery.
3) Saying the north was anti-slavery and implying the south was pro-slavery is misleading. I grew up believing (and still believe, to an extent) that the war wasn't over slavery. Or rather, it wasn't simply north = slavery good / south = slavery bad. For many I suspect the question wasn't is slavery good, but rather who should be deciding that question. And while I of course find slavery highly offensive, I also find it offensive that outsiders to the society should be imposing the decision on a culture.
Small comfort to the enslaved, of course. But I do wonder how many Southerners in the 1860s just framed the question differently. It's kind of like with the labels pro-choice and pro-life. Is anyone actually against choice or life?
1 A.M. blog posts aren't always the most thoughtful. But really, I am frustrated by the exposure and am really and truly glad that the anniversary has passed... :-S