Why did Chief Justice Roberts conclude that the individual mandate could not be upheld under the commerce clause?

Chief Justice John Roberts concluded that the individual mandate could not be upheld under the commerce clause because the power to regulate commerce presupposes the existence of commercial activity to be regulated.1

In the Affordable Care Act cases, Roberts argued that the failure to purchase health insurance does not constitute an activity in the ordinary sense of the term. He noted that the mandate does not regulate existing commercial activity but instead compels individuals to become active in commerce by purchasing a product. Consequently, he determined that the commerce clause does not license Congress to require individuals to purchase insurance, as it applies only to commercial activity and not to commercial inactivity.12

While he rejected the commerce clause as a basis for the mandate, the Court ultimately upheld the provision as a valid exercise of Congress’s taxing power.13