The big shift came in 1937, when the Supreme Court upheld the National Labor Relations Act. The Court made it clear that a democratic effort to stabilize labor-management relations in any particular area would inevitably have effects on the national economy. The Court learned this lesson, involving the economic interrelationships of the states, very well. Between 1937 and 1995, it never struck down legislative action under the Commerce Clause. In the midst of the Depression, the nation required concerted federal action, and the Court could not long stand in the way.
Economic interdependence is inevitable in modern societies.
If a strike occurs at a plant in Ohio, it is likely that sooner or later, people outside Ohio will be affected. If there is racial discrimination at hotels in Georgia, it is inevitable that some people will be less likely to travel and do business in Georgia. The level of wages for low-income workers in New York will matter to states other than New York. Seemingly local acts often have national repercussions. In any case, there is little need for an aggressive role by the Supreme Court in protecting states against national incursions.