The high-altitude airports have thinner air. At any particular aircraft speed, the thinner air means less lift compared to a flying into or out of a lower-altitude airport. To compensate for that, longer runways are used to achieve greater speed for takeoff. When landing at a high-altitude airport, the airplane touches down at a higher speed than normal to maintain lift, so the longer runways are also helpful for landings.
Work Step by Step
This is discussed on page 274.