One of the things about Australia is that Asia is nearly impossible to take, so typically, no one tries, which means people have to go pretty far out of their way to mess with you. Not so with South America. Both Egypt and Northern America are maintainable, which means that the South America guy has to make at least one enemy or hope they have something elsewhere to keep earning cards.
If you already have Australia, Asia is also defensible with only 3 points and if you can keep that even once, the game is basically over.