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.
I wouldn’t say that I was leaving money on the table. There’s a difference between gross income and net income - my gross income would have increased if I rented my house out, but my costs would have increased as well. Once I accounted for the cost of my time, of the additional maintenance, and of the risk that I would get someone who refused to pay rent, trashed the place, and took a year to evict, I decided that the net income from renting would be negative.
I was just a guy with a house, not a professional landlord. I think a professional would have found a profitable way to rent out a house like mine (because it wasn’t rent-controlled). But these vacant properties are already owned by professional landlords - I’m not sure who you think would find a way to make money off of renting them out if the professionals can’t. And why would someone buy a property that doesn’t make money?
(I suppose some people would buy them for a low price as a bet that the restrictive regulation will be repealed someday. Where I live, you can already get cheap rent-controlled property in very expensive parts of town with the hope that you might eventually be able to get the tenant out of there. But I don’t see this as a viable large-scale solution.)
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.