This question logically speaks two answers. 

The International Civil Aviation Organization (ICAO) along with many country’s aviation authorities address and limit operations over high altitude terrain.

If a passenger aircraft loses one of its engines for any reason, although it is still able to safely fly, it can no longer fly at a very high altitude. Thus it needs to “drift down” to a lower altitude where it can then maintain a safe and stable flight despite the lost engine.

These drift down procedures to a predetermined (x) altitude due to an engine out and respective gross weights were published, and available to pilots and their airline dispatchers. In mountain areas, this ‘safe’ prescribed altitude would sometimes be lower than the mountain peaks, and thus not an available flight route.

Another reason which has less acceptance is that the route doesn't generate a considerable revenue through passengers. But this reason was ruled out by many Aviation techies.