Not sure if you are aware, but all forms of interest expense is tax deductible for corporate income tax purposes in the United States, not just mortgage interest. This conforms to codes in other countries. There is no subsidy that is specific enough to rentals or investment properties. And since most of corporate bank lending is secured by all assets (including real estate), mortgage debt is somewhat fungible with other secured corporate debt, so it would be pointless to try to tax one but not the other.