Nothing, really. I personally find car racing to be a bit boring, but I don't have anything against it. I just don't think NASCAR is at a higher level than the NFL in any apples to apples comparison. At the same time it beats the NBA all to hell.
I agree with you.
It could be there someday if two things happen: 1) Fantasy NASCAR catches on; and 2) Gambling on NASCAR takes off.
Take fantasy and gambling away from the NFL, and the popularity drops substantially.
With regard to racing, if you have a favorite driver and focus on the little things that make up the big picture in a race,(points, driver rivalries, drafting, handling, tire wear, pit strategy, etc.) the four hours of a cup race goes by in no time flat.
If you look at it as "big pack of cars going around in circles," I could see how it could get boring.