i would say it's because a lot of rich people generally prefer to live there
with their relatively high purchasing power, they drive up the costs of certain commodities and services for everybody else in the area.
the costs of commodities and services in particular like.... housing... medical care... university education... are a lot more prone to be driven dramatically higher, by the demands of rich people, than various other, less vital ones, like fast food and those cheap little foam rubber balls you squeeze in your hand to try and stave off strokes and high blood pressure from the stress of your soul killing grind of a thankless, low paid desk job, in whatever random, kafkaesque corporate bureaucracy you manage to scrape the bare minimum required to keep yourself from freezing and starving, as long as you don't insist on actually getting sick, living in a house, or getting an education. not that i'd know anything about that.
but anyway, what with this "increasing disparity in the distribution of wealth across social classes" i hear has been going on around here, since about, oh, 1979, i don't think it's too surprising that you see a little more of this kind of thing, nowadays, in various places.
but that is just my opinion