Final answer:
The Supreme Court ruled in Dred Scott v. Sandford that Dred Scott remained enslaved because Blacks could not be U.S. citizens, residence in a free territory did not confer freedom, and Congress couldn't limit slavery's expansion, making the Missouri Compromise unconstitutional.
Step-by-step explanation:
When deciding that Dred Scott would remain enslaved, in the Dred Scott v. Sandford decision of 1857, the Supreme Court, led by Chief Justice Roger Taney, ruled against Scott. This landmark case established several key points: firstly, the Court decided that Black people, whether free or enslaved, could not be considered citizens of the United States and thus lacked the standing to sue in federal court. Secondly, the Court ruled that residence in a free territory did not grant freedom to an enslaved person upon their return to a slave state. Lastly, the ruling stated that Congress did not have the authority to prohibit slavery in federal territories, thereby declaring the Missouri Compromise unconstitutional and invalid.