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 incentive is that the building was built with a grant from the government to help initial building costs with the promise that it would be section 8 for X years, but section 8 doesn’t get landlords rich, so they look for loopholes to get around it. One of them is keeping the building vacant for those years and then “renovating” to make more money off of the tenants that aren’t section 8.