To make housing more affordable in expensive areas, there are a few choices. You allow more development and hope supply increases lower prices. You can subsidize it. You can require that developers include some below market rate units. Or you can allow or require that the units built are "worse" and therefore cheaper. Worse doesn't have to mean it's going to fall apart, but it does mean fewer attached amenities. In fairly dense urban areas, parking is a costly feature that new residents are required to pay for so that older residents can maintain their free or cheap on street parking spots. Mandating that parking spots are attached to a unit increases the cost of construction, in the same way that mandating granite countertops and expensive European kitchen appliances would increase the cost of construction.
More generally, all kinds of housing codes increase the costs of construction. I think (too lazy to verify right now) that new units in Philly have to have sprinkler systems. This might or might not be good policy. Above my pay grade. But it does increase the cost of building, as do setback requirements, minimum lot sizes, height restrictions, and, yes, parking.
