Final answer:
The Supreme Court ruled against Dred Scott and denied him his freedom because slaves were considered property, not citizens.
Step-by-step explanation:
The Supreme Court ruled against Dred Scott and denied him his freedom because slaves were considered property, not citizens. The court held that African Americans, whether free or enslaved, were not and could not become U.S. citizens. This meant that they did not have the right to sue for their freedom in federal court.