Because thinner atmosphere makes less resistance
Based on temperature the atmosphere can be divided into 4 main regions.. troposphere, stratosphere, mesosphere, thermosphere.

From the surface upto an altitude of abt 10-16 Km, the temperature decreases with increase in altitude. This is because here the atmosphere is almost transparent to the sun's energy & is instead heated from below, by the earth's surface. This region is the troposphere.

Above this lies the stratosphere, where the temperature increases with increase in altitude. It extends to an altitude of about 50Km. Airflow in the stratosphere is much less turbulent than in the troposphere. Here the air doesn’t flow up and down, but flows parallel to the earth in very fast moving air streams. This is the layer where most jet planes fly.

Due to difference in their temperature gradients, the tropospheric air does not mix into the stratospheric air. The lack of mixing & less turbulence makes the stratosphere more layered or 'statified'.