How and why are the Sherpas more able to endure the extremes of the environment, unlike most other people?

They have evolved to endure more as well as having been exposed to it from birth.

The native Nepalese Sherpas have been residing as a dominant community in relatively high altitudes near the Himalayas for generations now. This has evolved their bodies to make the most efficient use of oxygen available in the thin airs of Himalayas. That being the reason, Sherpas can endure the extremes of high-altitude weather better than most other people.

Explanation:

Sherpas have evolved and adapted themselves to the surrounding so finely, that their bodies have learnt to resist the scarcity of oxygen by making the most efficient use of available oxygen. Their bodies have also attuned themselves to preserve and burn calories for a good long time.