It's because the housing is highly subsidised by the government, in an attempt to prevent gentrification pushing low income people out of the area, breaking up

It's because the housing is highly subsidised by the government, in an attempt to prevent gentrification pushing low income people out of the area, breaking up their communities, changing the ethic makeup of the community and other such social issues. The government does not want the limited number of subsidised spaces to go to people who are happy to move away from the area and receive the market value of renting the place. This effectively results in the government granting them an income, and the social problems continuing, where a space would otherwise have been available for somebody else.