I am not seeing why the North couldn't just impose the end of slavery on the South, but not annex the South. I mean... if they had done that then it would be clear the war was about slavery. Having annexed the South back it becomes logical to suppose this was more about US economic might, or at least that this was a core part of the civil war (ie a part without which there would be no war). Besides, the South included major rivers-- supposedly the US would even declare war on France to secure one of those rivers prior to just buying it from Nap.