I didn't know this part, explains a lot: "But then USAC made the first of several fatal mistakes. In 1971, it removed the dirt races from the national championship and created a separate Silver Crown series. That didn’t have an immediate affect on attendance as Foyt, Andretti and Big Al still ran dirt races for a few more years but it gradually took its toll. Without Indy’s big names competing, the crowds began falling and national television no longer had any interest.More importantly to USAC’s faithful, it cut the artery to the Indianapolis Motor Speedway for the future generations of midget and sprint car drivers because the versatility of running dirt and pavement was no longer required in IndyCar. ..."