I disagree that it's a "young people have shorter attention spans" thing. Young people have long attention spans for lots of things, like video games, different TV/Netflix series, and other sports with long games like soccer.
There are a couple factors at play here, and it covers all age demographics. First of all, we have almost infinitely more options today than in past generations, in terms of how we want to spend our screen time. It used to be that your screen time on Sunday was to watch the race or golf or football, or the news, or some syndicated old sitcom.
Now you can watch whatever sport you want, whatever TV show you want, whatever movie, youtuber, any interest, any hobby, you name it. Plus you have social media engagement that flatout didn't exist and also eats into our time. So it's not that young people now don't have the attention span. They have the attention span - they just don't need to settle for something they're not 100% into, and there are so many other things competing for everybody's attention than ever before. NASCAR needs to be as compelling as anything else if they want people to bite.
That leads me to my next point. It isn't as compelling to sit on the couch for three to four hours to watch a race unfold because races don't unfold the way they used to. Stages create cookie-cutter strategies almost every single week. We used to see tons of variation there. We also don't have random mechanical failures like we used to, which takes an element of surprise out of the event. There aren't as many variables to each weekend as there once was, so it feels pretty repetitive these days. I can totally see how somebody that isn't emotionally invested in this for several years already would have a hard time all the sudden committing a night or afternoon of every week for the better part of the year to this.
It flatout isn't as much of a developing story as it was in the past, and people want that story. That's what grabs people. Leaders staying out to win the stage and others pitting two laps before the stage end, every, single, weekend, is not what people are going to turn their undivided attention to when they have literally endless possibilities available to them. That affects *everybody* today.