Answer:
Because there is less overlying atmospheric mass as elevation increases, atmospheric pressure decreases as elevation increases.
Think about what happens when you are on an aeroplane. As you descend [going in for landing], atmospheric pressure increases again from when it FIRST started decreasing when you took off. That is the "popping" feeling inside your ears, which requires the chewing of gum.
I hope this sparked a general ideya inside you, and as always, I am joyous to assist anyone at any time.