It seems that there was a period in the 1960s through the late 1970s where it enjoyed substantially more mainstream support in the US than in does today, and a higher level of American participation in many forms than has been seen since. For a brief time, it was popular enough that it put on two Formula 1 Grands Prix, Steve McQueen starred in a movie about the 24 Hours of Le Mans, and a string of fairly successful American F1 drivers. There were a number of highly competitive American MotoGP pilots from the late 1970s through the early 1990s and the Indy 500 allegedly had sell out crowds for decades until the mid 1990s.
It seemed like wherever there was an event that involved making a vehicle go far faster then reasonable, Americans were in the thick of it. Starting in the early 1980s with Formula 1 and gradually continuing to other series and disciplines, it seems as though this evaporated. Now NASCAR and Indycar, seem to be the only two "big" series to maintain a high level of American participation, and even these seem to have dedicated fans in particular regions of the country, with next to nothing in others (ie, the South and Midwest vs MA South through NJ). Smaller series like IMSA, the USAC, and NHRA are hanging in with some American participation, but these seem even more niche than NASCAR and Indycar.
What prompted this massive exodus of Americans, both fans and athletes, away from the wider world of motorsports they once seemed as if they couldn't get enough of? And what was different about NASCAR and Indycar that enabled them to endure throughout this same time to enjoy relative success today while other series find little purchase in the US market?
No offence, but I'm not sure I'd agree with the premise of the question. Until 1979, when CBS showed the Daytona 500 live, auto racing was featured on ABC's Wide World of Sports but rarely live or flag-to-flag. The Indianapolis 500 wasn't even shown live - it was taped and broadcast that evening. Having every NASCAR, F1 or Indy Car race broadcast on television was just a pipe dream.
I'd argue that racing became more popular in the eighties and nineties, as cable networks (most notably ESPN) began showing every race. American participation in F1 began to decline as the CART Indy Car series became more popular.
Then came the CART/IRL split of the mid-nineties, which NASCAR brilliantly exploited to become America's favorite racing series. Once a Southern regional sport, NASCAR expanded to tracks in places like New England and Chicago. The series enjoyed by far its most mainstream popularity in the 2000s, and only in recent years (since the 2008 recession dried up many sponsorship opportunities, as many top drivers like Dale Earnhardt, Jr. have retired, and as NASCAR can't seem to stop fiddling with its rules) has its popularity begun to decline again.