An ACW game won't be made in the near future for the commercial reasons that others have given, but I also think that there are three reasons which make it quite unsuited to the Clausewitz engine with the grand strategy mechanics we know and love.
Firstly, there's really only one possible conflict and limited replayability. In EUIV, you can play as France and decide to go colonial, or expand into the Med, or turn Protestant and reform the HRE, or.... Even in HoI, there are multiple possibilities. Japan can go north into Siberia, south into the Malay Barrier, or sink everything into China. Germany can try to finish off Britain before invading the USSR. The British Empire & Commonwealth can focus its efforts in northwest Europe, the Near East or the Far East. What would people say about an ACW game where the Union decided, "OK, you can keep slavery if you want to, we're off to fight Canada"?!!!
Yes, there is the possibility of the British Empire, France, Mexico, and maybe even Spain joining the war, but they would have to be mainly off-map powers (like China in CK2 Jade Dragon). How many times would people have replayed Street Fighter if had been the same two characters every time, or Civilization if it had only had two civs?
Secondly, as others have said, those two sides were so imbalanced. I'm sure someone will make a mod for HoI 4 eventually, but to be realistic it's going to have to give half a dozen factories to the South and fifty to the North. The South's strength was in cotton production and I've yet to see a wargame where that featured heavily. For the South to survive it needs exceptional generalship, which is not the AI's strong point. You can give the South general characters with better attributes, but it means that the player is set up to either win or lose in advance, which isn't fun at all. With only two imbalanced sides, there's no possibility of any rock-paper-scissors dynamic to keep players interested.
Thirdly, the two sides' strategic choices are relatively limited: advance along the coast, advance up/down the Mississippi, advance along the Tennessee valley.... The real interest in the ACW is more about tactics. Although I haven't played them, there are apparently some very good tactical games for the ACW, such as Ultimate General, and Sid Meier's Gettysburg! But anyone who's ever played a Clausewitz game knows that's it's not about tactics, even in HoI; that's why individual battles play out in minigames that you can only influence to a very limited extent.
These issues didn't apply to March of the Eagles or Sengoku, which both featured environments with multiple competitive countries/daimyos and many possible diplomatic and military strategies. Runemaster tried to use the Clausewitz engine for a non-grand strategy game and never reached a playable state, I believe.
There's nothing wrong with ACW games, which can sell well in the US, but a Clausewitz GSG isn't the way to do it.