Because while the Republicans were willing to accept slavery where it already existed, they were completely opposed to the expansion of slavery into the territories. Nothing, not the Corwin Amendment or the Republican Administration, guaranteed that a southern slave owner could move to any territory of the United States with his chattel. So the south took matters into their own hands, announced their secession, adopted a constitution that not only protected slavery and slave imports, but also guaranteed that any future states and territory would be slave, and launched their rebellion.
There you go. The mudslingers would have us all believe that the North was such a nobel enterprise that it fought the evil slavers to the South to end slavery. So it wasn't to end slavery at all then. It was to prevent slavery expasion to the West. And why would the North want to prevent such expansion? Purely humanitarian reasons?
You are quite wrong in stating that the Confederate Constitution protected slave imports. It actually banned them from all foreign countries except the United States and its territories. See Section 9 of the Confederate Constitution.
And, to paraphrase Jefferson, a little rebellion now and then is a good thing. Heaven knows we could use some rebellion against the socialist policies imposed upon us by the representatives and senators elected by the blue states.