The federal Department of Housing and Urban Development (HUD) contracts with for-profit owners of private multifamily housing [to] make specific rental properties available to qualified low-income households...Approximately 1.2 million renter households of all ages benefit from this type of rental subsidy, including 500,000 older households. Most Section 8 contracts have expired or will expire soon, and the property owners must now decide whether to renew their contract or leave the program ("opt out"). The problem is concentrated among those projects for which the rent levels set by the Section 8 contract are below the prevailing market rents for comparable units. Owners thus have an incentive to leave the program and convert their property to private market rentals.
—from the Executive Summary of the AARP Public Policy Institute Issue Brief by Andrew Kochera, Don Redfoot and Jeremy Citro (15 pages)