Why did Chief Justice Roberts reject the necessary and proper clause as a basis for the individual mandate?

Chief Justice John Roberts rejected the government’s argument that the individual mandate was constitutional under the necessary and proper clause because, while the mandate might be "necessary" to support other provisions of the Patient Protection and Affordable Care Act, it was not a "proper" means of doing so.1

Roberts reasoned that accepting the government's position would unduly expand federal power. He argued that such an interpretation would enable Congress to reach beyond the natural limits of its authority and bring individuals who would otherwise be outside its regulatory scope into its reach. Consequently, he concluded that the mandate could not be sustained as an "integral part of a comprehensive scheme of economic regulation" under that clause. Instead, the Supreme Court ultimately upheld the mandate as a valid exercise of Congress's taxing power.12