The way GST is applied to purpose-built rentals.... In most situations, when businesses are paying GST, it's meant to flow through to the customer, and the business will get its GST refunded through input taxes, but of course there is no GST applied to rental. When you charge GST to people constructing purpose-built rentals, there is nowhere at the end to get the input tax credits back from, so that just ends up buried in the cost of rent. That either makes rent more expensive or makes it a less desirable investment from the private sector to build more purpose-built rentals.
Unlike groceries, which are considered a mandatory essential and there is no GST, and so accommodations are made for grocers in that chain, that isn't happening in the purpose-built rental market.