Food for Thought....The Last Generation?
Stock Car Racing History
Like Jeff said, a lot of fathers stopped watching, which will affect their sons following. But apart from that, you just have too much else to keep your free time occupied these days compared to the past, whether you're little or grown up. Back in the day, you had hardly any channels on TV, you had no internet and all the Netflixes, Youtubes, video games, facebooks and what not that come with it..
..and last but not least, you had far fewer alternate forms of motorsport itself that you could follow outside your region. Now you've got series that get over sooner, offer more excitement for your time and money and you can watch from any part of that world, so more and more people would rather watch that instead of a 4 hour marathon where you're simply logging miles for 3/4 of it.
Also has to be said that car culture is fading too. I don't think as many people work on their own cars anymore. And the cars most of them drive are not represented in NASCAR. The younger generation NASCAR wants so badly is driving hot hatches instead of big 4-door sedans.
So the sport will live on in some formand while we're not the last generation of its fans, it'll be smaller generations from here on out.