Why? Because apparently they need some more incentive to keep units occupied. Also, even though a property might be vacant, there’s still imputed rental income there. Its owner is just receiving it in the form of enjoying the unit for himself instead of receiving an actual rent check from a tenant. That imputed rent ought to be taxed like any other income.
What you are describing is the same problem though. If you sell it and someone comes in and renovates it for use as a primary residence, then it is no longer affordable housing.