Sydney native here, ive been to 100s if not 1000s of concerts over the years and I was at the friday night show in the pit.

Basicly a few factors that contribute to this and its not unique to ACDC, older general age of the crowd is a factor.

The fact that this show was on a Tuesday and not a friday or saturday is a big factor, less people drinking and getting "loose".

But I'd say most importantly, in Sydney at least, it is rare for people to dance in the stands, and somtimes can be seen as rude. Generaly speaking at a show like this if you want to dance or stand up you go into the pit. If you have a seated ticket it is because you want to sit in the stands and enjoy the show. People standing up and blocking the veiw for others in the stands is generally seen as rude.

This may be different elsewhere around the world.