If I had to guess I'd say the demand is for "housing within N kilometers of point X" and there is nowhere left to build housing in that area, whether because of physical constraints or legal/regulatory constraints (e.g. can't get a permit to demolish that block and put up a dense high rise).
(Edit for non-USA-ian units, since we were talking about Geneva.)
(Edit for non-USA-ian units, since we were talking about Geneva.)