As the constitution explicitly protected it according to the laws of any state that had it, the more salient question is by what constitutional power could anyone claim to legitimately prevent it from going into the territories?
If it were legal in the States, and protected by the Constitution, by what claim can it be made illegal in non states?
But apart from that, the theory that slavery was going to expand to the territories appears to be a lie. Large scale slavery such as plantation slavery was simply impossible in the territories. There was no farm-able land capable of producing the sort of cash crops that were needed to maintain it.
It could not have expanded to the territories to any significant degree because the lands in the territories could not support it. Here's modern proof of that. (everything in West Texas/and Oklahoma and all parts further west required irrigation systems that could not be built in the 1860s.)
I now suspect the entire fear of slavery expanding into the territories was an astro-turf effort to maintain the New York northern coalition's power in congress, because they had used this power to maintain the system of funneling the profits from all the Southern produced export products into New York city.
Allowing more representation in congress that would side with the Southern states would cause changes to certain laws that were funneling that money into New York, and the power people of New York and Washington did not want that money stream changed.
Same today as it was then. The New York/Washington DC spending coalition keep the money flowing through their hands.