From WaPo

The supply has not kept up with demand in part because of a shortage of apartments, a key source of new rentals. Developers cut back on such projects when the economy deteriorated in 2009, which drove down vacancies and boosted rents. Analysts say they expect rents to keep climbing as developers try to ramp up new projects and catch up with demand.

The scarcity of affordable rental units was most pronounced in the West, where only 53 units were available for every 100 very-low-income households that are looking to rent, according to the study, which analyzed federal survey data from 2009. That compared with 65 in the South, 66 in the Northeast and 87 in the Midwest.