Recently, there have been a lot of pictures coming out of the deconstruction grandstands. They are stands that were built when times were good and the sport was in a growth mode. Those days are gone and won't likely return.
The problem that all the tracks are facing now is perception. How can you convince casual fans and, more importantly, sponsors that your sport is worth investing their time and money in if huge swaths of empty seats show constantly in the background? That is the appearance of lack of interest.
There are plenty of good reasons that fans aren't turning out in the same numbers as they were, say, 10 years ago. And, it's not just a NASCAR problem. The economy, the cost of gas and the improvement in the at home viewing experience are all valid reasons that fans are staying away.
So instead of leaving the empty seats there and hoping fans will return someday, it makes a lot more sense to tear them down and, possibly, turn that space into a place RVs can park overlooking the track.
And, the side benefit is: if the fans should start filling your other seats, there would be more of a sense of urgency in getting them to buy their tickets earlier. Any promoter wants their tickets to be considered a tough ticket. Especially for outdoor sports, you don't want fans to feel like they can wait and see what the weather is going to be before deciding whether or not to buy.