The ranching in South America has pretty much the perfect conditions for it. There's abundance of grass (so no expenses for additional food), the climate is very suitable for the animals, there's wast spaces that allow this activity, the predators that can harm the animals like the jaguar and the anaconda do almost no damage, apart from very rare isolated cases.
The ranching in Australia is very limited in space, inland the country is arid, so only the regions close to the coast line are suitable, but not even all of them, the north is not suitable for that because of the rain forests. The grass is restricted in amount so there's plus expenses for additional food for the animals. The climate is not suitable in summer for the animals because it's very hot and dry. There's no predators that attack the animals (the dingo can, but doesn't), but the highly venomous snakes occasionally can cause damage.