After the death of Dale Earnhardt in the 2001 Daytona 500, NASCAR started to work on a new car to make safety the number one priority. They also worked on the walls, putting up a foam "safer barrier" to make the impact of the cars hitting the wall diminish greatly. Thus the G-forces that hit the drivers would be significantly reduced and as a result not cause injury or death, which was common in NASCAR for its entire existence.
And while most fans, myself included, agree that safety is truly important in stock car racing (they can now take hits at 180 mph head on to the wall, flip 8 times, and the driver not be injured at all), the result was something that really killed NASCAR. Which is ironic, because it has done so much for the safety aspect, thus reducing the death count to 0 in the last 10 years. The car, known as the Car of Tomorrow, or COT, was a truly ugly piece of machinery. NASCAR had since the 1980s been going further and further away from the Stock in Stock Car Auto Racing. Up until 2002, although the differences between the manufacturers was almost gone, the cars (from Ford and Chevrolet for example) had distinct differences that set them apart from one another. But in 2003 the cars all had the same body, minus some very small details such as the back window and the nose of the car.
They did have very small differences, but they still had at least some distinction. But with the introduction of the COT in 2007 all manufacturer resemblance was gone. All cars, whether Ford, Dodge, Toyota, or Chevrolet had the same shape and same body. There were no differences whatsoever. The different car companies could change the grille on the front, but that was it. The fans in massive numbers didn't like it. Even the drivers didn't like it. Kyle Busch won the first race, the first Bristol race in 2007, with the car and after he won said "they suck" in victory lane. The fans showed with their remote controls and ticket purchasing ability that they didn't like the car, and just quit showing up. The growth of the 90s and the distinct cars were gone. Dale Earnhardt was gone. The sport just wasn't what it once was. Many people, even longtime hardcore fans quit watching the races, saying it's just too different from the old days.