What happened? That is hard to say.
The sport perhaps had become too regional, perhaps money was tighter, there is far more programming available with the cable and streaming platforms reaching many more TV sets (and computers/phones) than ever before. The Old Boys network is dying out, certainly a great many things are killing the sport.
Not to mention burning fossil fuel. The 43 cars racing at this year’s Daytona 500 will have used an estimated 5,375 gallons of racing fuel, according to NASCAR.
The ticket prices, once well paid and high, are dropping as people are not willing to pay the higher prices, the idea that car racing (at least with fossil fuel burning engines ) is not as attractive as the new E-Sports…probably a combination of all of them.
In a way it is progress, thinning the herd, and delivering what people are willing to pay for. However you may feel about guys making left turns, I hope the soap opera of stock car racing continues for a good long time.