I think it boils down to 3 major points:
1- USA is not an European civ. for gameplay purpose.
The devs dit not decide to not put the CSA in the game on purpose: USA uses States to Age Up instead of Politicians + Revolutions.
2- Revolutions are not popular.
They would probably need to create 4 new Revolutions for USA (none of the old ones make sense… well maybe USA revolution…lol). I think most players would prefer they dont waste time on this and give us new civs instead.
3- CSA is still mostly about slavery
No amount of historical revisionism is going to change that fact.