Comfort from others who are similar, whole host of reasons. For some people moving abroad can be really daunting. Also, often people have no idea where in a city tp move to so if they have a family friend etc living in an area they will just go there, I've had relatives do the same in winthrop. There are also some things that appeal to particular groups, eg the Italians tended to live close to towns as thats what they wanted, even when no one else wanted to. Poms want to be near the water etc.
You're entirely wrong re your second comment btw. "White people" and indeed australians of all colour (not sure why this is a colour issue and not a natiobality one) already do that in expat communities around the world. Go to parts of hing kong or singapore and see how dominated by white expats they are, is that racist as you say or just easier for people ?
Also look at the strong percentage of English pepple living in say far northern coastalk suburbs, same thing.
remove the chip on your shoulder and you'll see there are clear explanations for this