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
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.