Its just because of the simple fact that most of the people are comfortable only with people from their own culture.

These kind of ads are mostly for part of villa or sharing etc. I don’t think real-estate agents advertise with nationality preferences. I once saw such an ad and asked the guy the same question.... he told the landlord of the building thinks that only That particular nationality people will take good care of his property and wont do any illegal things in the premises. He said that from his experience.