Why planes do not fly over North Korea: Explained

North Korea: If you look at the online flight map, it is easy to notice that the airspace over North Korea is mostly clean – almost no one flies over it. But why?

This is understandable, given the high degree of closeness of this country to the outside world. The authorities forbade the aircraft of other countries to cross its airspace, so she has the right to shoot down any civilian and military objects appearing in the air.’
At the same time, citizens of some states can enter North Korea, they even sell tours there from Russia – how do people get there? There is usually an exception to each rule, in this case, the airline is Korean Airlines (Air Koryo). 
Only this national air carrier is allowed flights over the territory of the country. Air traffic in the DPRK, to put it mildly, is not very active – only 3-4 flights a day: therefore, the map of aircraft movement in this region is almost always clean.
Since both existing answers are a bit incomplete, it is worth discussing what FlightRadar24 itself has to say on the matter. In principle, they have coverage from receivers positioned near the borders of North Korea.

Wireless signals do not care about border restrictions, and all commercial aircraft are equipped with a transponder (SSR/ADS-B) which can be received up to 500 km or more. At least in higher altitudes, where line of sight communication can be established, aircraft can be seen.

As the transponder protocols Mode A/C/S and ADS-B are standardized and entirely unsecured (i.e., no authentication or confidentiality is offered), there is also no way that these commercial flights could selectively not cooperate with “the West”. It’s binary, either your transponder is on or it is off for everyone, and the latter is not an option in controlled airspace.

So, the short answer is true that there are simply not many flights. Only a hand full of mostly Russian and Chinese air carriers use the North Korean airspace.