Agree with Rhys. Not sure if it comes down to money alone. Look at the initial Festival at HD (Bruce McLaren). There were good crowds. The media were hyping it up for weeks or even months prior to the event. On the Friday morning immediately before the event, breakfast TV broadcast live from HD. There was an air of excitement not seen for some years. As time has gone by the numbers have declined.
Go way back to the NZGP days at Ardmore. The biggest crowds ever seen at NZ motor sport events were in attendance (60,000+). Bear in mind that the amount of discretionary spending money of most people was considerably lower and that travel was much more difficult than it is today. Yet large numbers of people travelled from the provinces to the GP even though it could require a tedious two day (or more) journey each way.
Last weekend the GP at Manfeild reportedly had an attendance of 8,500. Of those, 5,000 were there on free tickets given to Palmerston North residents! I believe 60,000 were entitled to these free tickets. There is something DRASTICALLY wrong when you literally cannot give tickets away to people who have to travel a mere 15kms to watch our premier motor race meeting!