The Tibetan Plateau is the roof of the world. While most people define it by the epic ranges to the north and south (Kunlun Mountains and Himalayas), it’s not immediately obvious that the “low” areas average above 12,000′.
There’s an issue with the very high mountains and losing an engine, but under most conditions, airliners can still fly higher than 20,000′. That’s not going to help over Mount Everest, but with the gradual nature of losing the altitude, it’s rarely a big deal.
The primary problem is that in case of decompression of the cabin, many airliners only have 20 minutes of passenger oxygen. Upon being presented with the rubber jungle, those neat yellow cups are only going to help for a limited time.
In consideration of the passenger conviction that air is nice, there are flight rules in place that force operators to be able to descend to 10,000′ prior to running out of emergency oxygen. Obviously, this part of Asia is a huge region and there would be a large swath that would be impossible to escape so quickly.
On top of that, there would be several points that would require the airplane to climb even higher to get to a suitable alternate field, exacerbating the situation. Fortunately, the pilots have a lot more oxygen, but the company frowns on arriving with 200 blue corpses.