Because the Confederates were committing treason in defense of slavery.
Be sure to read the article Lest We Forget Why They Fought, an excerpt of which appears below:
So if you are from the South, you have no need to apologize for the Confederacy. Even if your ancestors include men who fought and died for the Confederacy, this is not a matter which ought to cause anyone today to evaluate you as any different than anyone else. You are not your ancestors. You have to make your own choices, and one of those choices includes deciding whether or not to be proud of a Confederate ancestry. If I had Confederate soldiers among my ancestors (I don’t think I do, but you never know) I’d say I respected their bravery, and that I understood why they might have thought they were fighting for their country. Nothing wrong with that, I suppose.
But at the end of the day, they were fighting for a morally indefensible cause and while I might prefer to remain silent about that, if forced I would have to admit that yes, I thought they were on the wrong side of the war. Treason in defense of slavery is not a subject matter appropriate for any freedom loving people to celebrate. The Civil War had good guys and it had bad guys. The good guys were the ones who won.