Air Astana BKK - ALA & v/v
I suspect that you are on the right track
Maybe I should have said ....
3) Lack of "SUITABLE" alternates over the Tibetan Plateau / Taklimakan Desert
Getting a 757 onto a 6,000 ft dirt strip at 5,000M ASL with only one engine might be a bit tricky.
Not to mention getting it out again, accommodation, etc.
I'd still like to hear any other theories though.
I would even accept facts.