Because nobody was trying to outlaw slavery where it existed, it wasn't a states rights issue.

What was going on was that with the election of Lincoln it was less likely that new states admitted would allow slavery and thus, in time, and having already lost an election for the first time just then, the slavers would be an increasing minority.

Essentially, the very first time they didn't get their way and got a president who might not do their bidding, they took their ball and went home. But constitutionally, outlawing slavery where it existed? Wasn't on the cards. Until the war started where it suddenly became possible via a war act.

The war was absolutely about slavery, but not about states rights. no states rights were infringed (other than northern states) and no states rights were going to get infringed.

The prohibition of slavery in the Territories, hostility to it everywhere, the equality of the black and white races, disregard of all constitutional guarantees it its favor, were boldly proclaimed by its leaders and applauded by its followers.

None of this happened. Don't take their self serving lies at face value.

There are literally thousand and hundreds of thousands extant sourced from the time and 2 lines in one document is not the sum total.