Yes, and laws had been passed in (nearly) all that either slavery was outlawed or no NEW slaves could be imported. Slavery was being gradually eliminated and that’s what the rebel states were upset about. And the potential economic impact.

In March 1861, Alexander Stephens, vice president of the Confederate States of America, gave his view on the issue:

The new [Confederate] constitution has put at rest, forever, all the agitating questions relating to our peculiar institution — African slavery as it exists amongst us — the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution . . . The prevailing ideas entertained by . . . most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. . . Those ideas, however, were fundamentally wrong. They rested upon the assumption of . . . the equality of races. This was an error . . .

Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner–stone rests upon the great truth, that the negro is not equal to the white man; that slavery — subordination to the superior race — is his natural and normal condition.

It’s not accurate to say it was “only” about slavery, but it’s the best one-point answer you could give, and the biggest issue.

The issue is that there has been a giant movement to minimize the “we really wanted slavery” part are reframe it was a “state’s rights” thing which is highly inaccurate way to frame things and stupid.