As givemewings pointed out some aircraft have more O2 capacity for diversions over higher terrain. AIrcraft without the large O2 capacity would have 'no fly' zones to avoid the higher terrain where O2 capacity, or s/e cruise capability, couldn't ensure clearance with high terrain in the event of a depressurization or engine failure. Parts of the Andes are an example of 'no fly' zones for different aircraft types.