If dense housing is available, this drives down the cost of living/price of houses. If suburbian homes are less valuable, it is cheaper to convert.

Also, driving down the price of housing prevents EXISTING public spaces and parks from being converted into homes, as cities will be less incentivized to do so.

There is already a lot of public spaces that are being converted to houses, so preventing a conversion to urban sprawl is equivalent to converting back to a public space.