Since I don't have any money tied up in the sport, I haven't done any research on the subject. I won't claim to have a comprehensive list. An abundance of 'cookie cutter' tracks? The effects of the recession on people being able to afford to attend, or on sponsors being able to afford to support cars? TV broadcasts shot full of commercials? Expanding too rapidly, and assuming it would continue? Earnhardt's death? Dodge's exit? Toyota's introduction? Dodge's return only to exit again?
I'll give PARTIAL blame to the rules changes, but there are plenty of other factors. I don't think either of us has a statistically accurate way to measure the individual impact of any of them. My opinion is just semi-informed guesses and shots in the dark. We'd have to start by asking former fans why they made their choices. I will take a bet that NASCAR's marketing office did that, along with other forms of marketing research I can't even guess at.
And frankly, events and activities have always risen and fallen in popularity on a cyclical basis. I regularly read here that viewership is down across the board in sports. Tiger Woods took golf to numbers no one had seen since Palmer and Nicklaus. Current players are as good or better, but their viewership is down anyway without the sport changing. Look at the mid-80s when pro wrestling was everywhere. The mid- and late 70s were all about tennis. Almost nobody cares about boxing any more, and a hundred years ago it was the top form of entertainment in sports. Why there are no westerns on TV anymore? Oh, they were replaced by 'reality programming', which fell to police procedurals. Stuff comes and goes. My point is that neither of us can accurately, definitively say where NASCAR would be today if there had been no changes made in the last 20 years.