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.
Landlords do have to pay an income tax on property regardless of whether it is occupied or not, it is just that when a property is not being rented, it generates 0 income and 30% of $0 is $0.