Why is it harder to breathe the higher in elevation you go?
As you ascend up a mountain, the amount of atmosphere above you decreases and so does the atmospheric pressure. In other words, the higher you go, the less molecules of oxygen there are to breathe.
For example, at ~20,000 feet, atmospheric pressure is less than half normal (meaning at sea level) which means the Po2 is less than half and will not saturate hemoglobin. This can cause drastic consequences for the body (anoxia).