Why did Dred Scott claim he was no longer in slaved?

Dred Scott claimed he was no longer a slave because he had lived in free territories where slavery was prohibited, specifically in Illinois and the Wisconsin Territory. He argued that his residence in these areas granted him his freedom under the principle that living in a free state made him a free man. His case ultimately reached the Supreme Court, which ruled against him, stating that African Americans could not be citizens and thus had no standing to sue in federal court. This decision further entrenched the institution of slavery in the United States.