OK, let me rephrase it. It was not prevented by the Constitution.
I've challenged any number of these confederbees to prove that slavery was legally defined by the US government, feel free to prove me wrong by showing a law that defines what a slave is how one becomes a slave, and what crimes a person would have to commit to be enslaved?
The legality of slavery was upheld by any number of Supreme Court decisions, including the Taney court in every chance it got. The most famous example, of course was Scott v Sanford.
free dixie,sw
It was not defined by law.
It existed to be sure but it was not written as law of the land.
There I believe lies the difference, Lincoln offered the south a Chance to make it the law of the land, they refused and legalized it in the Confederacy.