Regional airports have become one of the biggest casualties of the coronavirus pandemic.

Aviation consulting firm Ailevon Pacific reviewed Cirium data to determine which airports the US' biggest airlines have left since 2020. The company's research revealed American Airlines, Delta Air Lines, and United Airlines have exited 59 airports.

In total, American has left 14 airports, Delta has exited 13, and United has departed 32.

While most of the airports are still served by competing carriers, a few were left without any airline service. Specifically, American has departed Dubuque, Iowa, and Williamsport, Pennsylvania, but neither has seen another operator yet.

Some of the airports are part of the government's Essential Air Service program, which was established to ensure certain communities do not lose their connection to the national air network by subsidizing airlines to operate routes to the protected cities. Dubuque and Williamsport are not on the list.

Pierre and Watertown in South Dakota, two cities cut by United in November 2021, are EAS airports. Denver Air Connection took over the service for United.

According to Ailevon Pacific's data, United has completed or will end service at 10 EAS airports by the end of 2022, including Alamosa, Colorado; Paducah, Kentucky; Fort Leonard Wood, Missouri; Kearney, Nebraska; Ogdensburg and Plattsburgh in New York; Shenandoah, Virginia; Clarksburg and Lewisburg in West Virginia; and Eau Claire, Wisconsin.

These routes have been replaced by carriers like Contour Airlines, Denver Air Connection, and Sun Country Airlines, per Ailevon Pacific data.

The trend of airlines leaving small markets is not new and became common during COVID-19, though the trend is continuing. American, Delta, and United have all cut routes in recent months, citing poor performance and the pilot shortage as contributing factors.

Henry Harteveldt, president and travel industry analyst of Atmosphere Research Group, told Insider in November that routes to many regional airports are simply unsustainable. 

"As hard as it is for the people that live in these small towns to lose airline service, it is an unfortunate reality that airlines are not just going to serve a city out of civic responsibility," he said. 

Harteveldt further explained that "airlines are going to seek out markets that they believe will give them an advantage, but if a city isn't profitable, they will cut it."

The lack of crews has also pushed airlines, including American and United, to ground regional aircraft because they simply don't have enough pilots to fly them.