Less atmosphere to trap in heat. The heat does rise but does not mean it is warmer. The earth's gravity pulls the air molecules in, the higher you go the less air molecules around you trapping less heat making it colder.
the earths gravity is not as strong, so the air pressure is lower. since the air is thin, there are less air molecules to hold heat. the individual molecules are hot, but the heat transfer rate is so low, that it only feels cold.