Loose rivets
mcgyvr81's excellent diagram and indeed, post, fits in frighteningly well with what I was trying to say.
If they had pulled the aircraft onto heading fairly tight in, that could have left as little as three miles to the point of impact. You can imagine the timeline and indeed the thought processes, as it became apparent there were walls on both sides.
Further speculation:
If they requested the descent for sightseeing in that area, there is also a great possibility that flying into that canyon was part of the intention. To circle a mountain only enjoys one part of the cabin, whereas the other part is looking at blue sky. The crew might have thought to be qualified for such a stunt (test pilots), the aircraft was light (short hop, no luggage, only half passenger load, probably not fully fueled) and lots of cameras for nice pictures on board.
When flying visual in such a valley, the rise of the floor and the therefore asociated climbrate is not noticed very well. As the obstacles are close, all senses are tuned to the outside of the cockpit, not recognizing the decreasing energy state, until it is too late and the terrain outclimbs the performance of the aircraft. The electronic gadgets (TWAS) would be turned of anyway if intentional flying that close to the ground was intended.
There is a great possibility that it was sightseeing, as a turn around this mountain in that altitude is not part of any kind of IFR procedure.
Aviaton is full of show-off accidents, therefore this speculation is not out of the world.