Why? It's understandable why we had to have a war with the United Kingdom because the foundation of British law did not recognize any right to deny allegiance to the King.
The founding document of the United States did however recognize such a right.
That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.
So why shouldn't a government founded on such a principle not recognize this same right for others?
Whether or not the CW was destined to occur is not going to be settled by reference to founding documents. The differences between the North and South were great, and people were angry. And, there’s no going back.
So, I don’t want to argue at any length about projections of alternate histories.
There’s only opinions, no definite conclusions to be reached. Especially in this case, where what’s done was done over 150 years ago.