"The Airline Deregulation Act of 1978 gave airlines almost total freedom to determine which domestic markets to serve and what airfares to charge. This raised the concern that communities with relatively low passenger levels would lose service as carriers shifted their operations to serve larger and often more profitable markets. To address this concern, Congress established the Essential Air Service (EAS) program to ensure that small communities that were served by certificated air carriers before deregulation would continue to receive scheduled passenger service, with subsidies if necessary. The EAS program is administered by the Office of the Secretary of the U.S. Department of Transportation (DOT), which enforces the eligibility requirements and determines the level of service required at eligible communities. By the end of FY2018, 174 communities in the United States received subsidized service under EAS."
CRS Report for Congress, R44176
Congressional Research Service: https://crsreports.congress.gov/