Why did the supreme court say that dred Scott was still a slave?

The Supreme Court ruled in Dred Scott v. Sandford (1857) that African Americans, whether free or slaves, were not considered citizens under the United States Constitution. As a result, Dred Scott was not entitled to claim his freedom in a federal court, and his status as a slave was maintained. This decision further aggravated tensions between the North and South over the issue of slavery leading up to the Civil War.