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.
The idea is that properties beyond the first get taxed more heavily. So if you’re going to buy an investment property, you better damn well make it attractive to rent.