you should read the constitution, it's clear as the sky is blue.
In an effort to preserve the balance of power in Congress between slave and free states, the Missouri Compromise was passed in 1820 admitting Missouri as a slave state and Maine as a free state. Furthermore, with the exception of Missouri, this law prohibited slavery in the Louisiana Territory north of the 36° 30´ latitude line. In 1854, the Missouri Compromise was repealed by the Kansas-Nebraska Act. Three years later the Missouri Compromise was declared unconstitutional by the Supreme Court in the Dred Scott decision,
The federal laws must be "in pursuance thereof" the constitution. Federal laws are also bound by the constitution. The constitution was a contract between the states and the federal government the states themselves had created. That contract was broken nullifying it.
I agree with you in regard to the states formed by the federal government on lands acquired by the Louisiana Purchase. Although there was a contract allowing slavery that was broken, they could only return to be a territory. But, a former territory aligning itself with another country recently formed by secession gets complicated.
Texas was an independent republic before joining the union and forfeited land above the 36/30 latitude to be able to enter the union as a slave state.