In many situations, the law of the state/territory in which an event occurs governs a case. In the Dred Scott case, the court ruled that Missouri law governed the case and that the Scotts remained slaves. It could more reasonably be argued that Illinois law or Wisconsin law should have applied because Dred Scott should under federal law have automatically been freed during his extended residence in a free state/territory. This is not a case where I am arguing for ignoring the Constitution and freeing all slaves because slavery is wrong.
I may be misremembering but I thought it was Scott that filed the suit in Missouri. The decision declared the Missouri Compromise unconstitutional so I don’t know that the venue was the determining factor.
I plan to read the Scott decision sometime - I think its 200 pages long. I’ve only read summaries but it seemed Taney - writing in the 7-2 decision - based his reasoning on original intent. Taney did own slaves but had freed them prior to the decision if I recall. I don’t think he was a strong supporter of the peculiar institution.