I wonder which policies and/or economic conditions discourage the building of housing for those people?

One over-arching one:

There is an unfortunate effect that once you are a homeowner, it is in your financial best interests to keep population density as a low as possible near you, as property values in any given location rise with lower density. As the homeowner population gets to a certain fraction of the population, zoning and building rules can't be made to their detriment. In most places anywhere near someone would want to live, it's impossible to get a permit to build any kind of higher density housing.

Needless to say, housing that most folks can afford is somewhat higher density. Whether rich or poor, land near where you probably want to live and work isn't cheap, so without a modicum of density, you'll always have a shortage of housing.

Jim