It's also because the lower atmosphere is far more turbulent than the upper, therefore it can fly faster and smoother while burning less fuel
Don't forget a very important element at about flightlevel350 :
JET STREAMS :
Jet streams are fast flowing, relatively narrow air currents found in the atmosphere at around 12 km above the surface of the Earth, just under the tropopause. They form at the boundaries of adjacent air masses with significant differences in temperature, such as of the polar region and the warmer air to the south. Because of Earth's rotation the streams generally flow west to east. The paths of the flows typically show a meandering shape, and these shapes themselves propagate east, at lower speeds than that of the actual wind within the flow.
There are two main jet streams at polar latitudes, one in each hemisphere, and two minor subtropical streams closer to the equator. In the Northern Hemisphere the streams are most commonly found between latitudes 30°N and 70°N for the polar jet stream, and between latitude 20°N and 50°N for the subtropical stream. The wind speeds vary according to the temperature gradient, averaging 55km/h or 35 mph in summer and 120km/h or 75 mph in winter, although speeds of over 400km/h or 250 mph are known. Technically the wind speed has to be higher than 90km/h or 55 mph to be called a jet stream.
Jet streams can be explained as follows. In general, winds are strongest just under the tropopause (except during tornados, hurricanes or other exceptional situations). If two air masses of different temperatures meet, the resulting pressure difference (which causes wind) is highest at those altitudes. If one of the air masses lies north of the other one, then the wind will not flow directly from the hot to the cold area as one would expect, but is deflected by the Coriolis force and flows along the boundary of the two air masses.
Airline great circle track at top, jet stream track at bottomThe location of the jet stream is an extremely important datum for airlines. In the United States and Canada, for example, the time needed to fly east across the continent can be decreased by about 30 minutes if an airplane can fly with the jet stream, or increased by about the same amount if it must fly west against it. On international flights, the difference is even greater, and it is often actually faster flying eastbound than taking the great circle route between two points.