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North Korea charges about $740 per flight, so cost is not the issue - the savings by reducing a flight path by a few hundred miles can be considerate.
Some of the reasons are due to South Korea, from the same article:
South Korea banned the use of North Korea airspace for aircraft coming in and out of the country's airports in 2010, following the torpedoing of the South Korean warship Cheonan.
So that affected some of them.
The lack of notification from North Korea for missile tests has also led to several airlines making their own no-fly zone around North Korea.
Same article also states:
The United States Federal Aviation Administration (FAA) has particular rules about flying over North Korean airspace. On April 18, 1997, the FAA issued Special Federal Aviation Regulation number 79 (SFAR 79), which said that “immediate action is necessary to prohibit certain flight operations within [North Korean] airspace.” The FAA were expressly concerned of civil aircraft being shot down by the North Korean air defence system. The FAA extended the ban to the entire Flight Information Region (FIR) of Pyongyang. This however changed in February 1998, when the FAA decided to allow flights in the eastern zone of the FIR.
SFAR 79 still applies to all U.S. aircraft. It bans U.S. air carriers or commercial operators from flying in the Pyongyang FIR west of 132 degrees east longitude. They can, however, fly into Pyongyang with approval from the U.S. Government or the FAA. This is usually performed in diplomatic, emergency or humanitarian missions.
Other international airliners are not as strict as the Americans. European airlines have used the airspace, including the regions banned by SFAR 79, for commercial purposes. Lufthansa, KLM and Finnair are some of them. In one video tracking air traffic over twenty-four hours, there is small amount of aircraft flying over the country. South Korea avoids the airspace at all costs. They intentionally fly in a strange so-called “dog-ear-style angular pattern” in order to avoid it.
Also, if you're watching Flightradar24.com for example, and don't see flights there, note that
ADS-B receivers are not allowed in North Korea and internet usage is restricted making it very difficult to install ADS_B sensors on the country. Site like FlightRadar and AirNav RadarBox24.com - Live Flight Tracker and Flight Status live out of this flight data so it is very difficult to get coverage there.