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.
Either the properties are valuable as rental properties, they’re not especially suited for rental properties, or they’re not fit to live in. If they’re in the first category, great. If they’re in the second, forcing a landlord to sell instead of sitting on an empty property helps, in that it creates more supply of livable homes for people making the leap to homeownership (that’s the answer to who would buy a house that doesn’t make them money: people who want to live in it). If the property is unsafe/unlivable, it should be repaired in a timely manner or torn down.