The Supreme Court cannot directly enforce its rulings; instead, it relies on respect for the Constitution and for the law for adherence to its judgments. Because the Supreme Court simply bases its decisions on the Constitution, the decisions are not overturned. The decisions simply uphold the Constitution but do not have outside enforcement.

Added: Short answer: (in the US) The Supreme Court is the highest court in the nation. Its rulings cannot be overturned unless done by a subsequent ruling of the same court.