It's also possibly a sign that riders are pushing harder. I am no great long time vintage racer, but in the 10 years I've been doing it I've noticed the standard of both bike presentation, and rider skills/aggression, go up considerably. When I first rode early HEAVEN meets, there were a sprinkling of fast guys and everyone else was pretty much pottering around (which is a worry as even then I couldn't keep up!).
But as you say, in NSW it reflects the tracks too - Canberra, Kembla, maybe even Glenbawn, which are the usual tracks raced and which have plenty of jumps and stuff. I find my '75 RM, even with top quality YSS suspension, is still a handfull at Canberra. And it hurts landing off some of those jumps!
I look at all those pics of lush green paddocks in Victoria, WA, and NZ, with a lot of envy...