The 'why' is pretty much unknowable. Evolution doesn't push organisms to do new things, it simply responds to existing selective constraints. What we observe is

The 'why' is pretty much unknowable. Evolution doesn't push organisms to do new things, it simply responds to existing selective constraints. What we observe is that Humans (even archaic ones) seem to have expanded into virtually every region they could find, regardless of how hospitable it was. This particular set of changes happens to reduce physiological stress at high altitude, so it was preserved in populations experiencing that constraint.