Because the Aboriginals were forcefully removed from their families (where they did belong) and were dumped into assimilation camps (were they didn't "belong")
Also the story of the Aboriginal people is that they have been dispossed of their own land and culture. This is the case with Rabbit Proof Fence because they are forcefully removed and put into a place where they clearly don't belong. They struggle to get back to where they truly belong and that is home.
They used to belong to the land, however now they are just strangers in their own country.