In my city, the average family income is about $60k but housing is expensive to buy. One-bedroom condo's start at $200k so a lot of people rent. Unfortunately, since developers can earn so much by selling units instead of renting them, they all veer towards that.

I'm sure rent control plays a factor too. Rent can only be increased by 3% per year or something like that.

End result, there hasn't been a new building built for rental apartments in my city for over 30 years.

Renting regulations are pretty notorious in Switzerland. There seems to be a rule for everything & it is notoriously hard to get rid of a bad renter. Also, as a foreigner, it is often difficult to acquire property in certain kantons.

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.)