There is no such thing as "no one else was willing to pay for a use of that space". Maybe no one else would pay the exorbitant price they are sold for and, most likely, no one would pay that price if they had to pay reasonable property taxes on that unit. They are doing no favors to the city. Since property taxes are so low, it often pays just to withdraw units from the market to inflate prices further by creating an artificial scarcity.