There isn't - among historians.

In the general public, it generally comes down to two things: identity and politics. You may be interested in some of the answers on the so-called Lost Cause of the Confederacy that can be found in the FAQ.

Long story short, former confederates, after the war, tried to white-wash their image, saying that they had not, in fact, seceded from the union to protect the institution of slavery. Now as I mentioned, there is absolutely no debate about this having been the case. They said it at the time, before secession and during the war, but after having lost the war, and seen emancipation, that reasoning was no longer politically sound, so they invented the Lost Cause myth. That's the major part of the politics. The identity part comes in because folks, primarily the descendants of former Confederates, have issues with taking pride in their ancestors and so tend to cling to the false notion that the war was not about slavery. After all, how can you take pride in your ancestors if they fought to uphold something that is pretty universally considered to be wrong? For whatever reason, there is a disconnect that occurs among some folks, wherein they cannot separate being proud of the martial spirit of their ancestors who believed for what they fought and being proud of the reason itself. Because of this disconnect, folks tend to argue that the reason for the war itself was something different (states rights, economic issues, what have you) than what it was primarily about - the maintenance of the institution of slavery.

Edit: added FAQ link. Edit 2: I cannot spell and spell check betrayed me. Edit 3: Obligatory "Wow! Gold? Thanks, kind stranger" edit.