Rental housing is a market and just like other markets it can be modelled using the economics described by the demand curve.
The demand curve maps the relationship between supply, demand and price.
It says that for high demand and low supply the price is high, for low demand and high supply the price is low and over time supply, demand and price will find an equilibrium.
So if you consider the housing market, on the supply side you are looking at a constrained resource (i.e. it is constrained by the land available to build).
The demand side will be driven by the numbers of people looking to rent and that will be driven by many other factors like work prospects, quality of life, crime rates etc etc.
So for places like SF there will be great demand for that limited housing which means the price (i.e. the rent) goes up.
The demand curve maps the relationship between supply, demand and price.
It says that for high demand and low supply the price is high, for low demand and high supply the price is low and over time supply, demand and price will find an equilibrium.
So if you consider the housing market, on the supply side you are looking at a constrained resource (i.e. it is constrained by the land available to build).
The demand side will be driven by the numbers of people looking to rent and that will be driven by many other factors like work prospects, quality of life, crime rates etc etc.
So for places like SF there will be great demand for that limited housing which means the price (i.e. the rent) goes up.