Based on information in the majority opinion, the supreme court decided that Fred Scott was not a free man because

Fred Scott was not considered a free man because he was a slave and he sued his master also to get freedom.

Explanation:

Slavery was at the foundation of Dred Scott's case. He sued his lord to acquire opportunity for himself and his family. The contention he utilized was that since he had lived in a domain where bondage was unlawful, he would never again be oppressed.

This was a regulation that was perceived in customary law for a considerable length of time in Europe.