Water boils at a lower temperature at high altitudes, because

Boiling occurs when a liquid's vapor pressure equals the surrounding atmospheric pressure [3]. At high altitudes, the atmospheric pressure is significantly lower than at sea level because the air is less dense [3]. Since the external pressure pushing down on the liquid surface is reduced, the water molecules require less kinetic energy (and thus a lower temperature) to overcome the ambient pressure and escape into a gaseous state [2]. Consequently, the boiling point decreases as altitude increases [1]. For example, at the summit of Mt. Everest, the air pressure is about two-thirds less than at sea level, causing water to boil at a much lower temperature than the standard 100°C [3]. While outside temperatures are often lower at high altitudes, this environmental factor does not determine the boiling point itself; rather, the physical property of the boiling point is directly governed by the ambient air pressure [2].

Sources

1. [3] https://www.sciencedirect.com/topics/engineering/boiling-temperature
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > Vertical Variation of Atmospheric Pressure > p. 305
3. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 3: Geological Time Scale The Evolution of The Earths Surface > 3. Geological Time Scale – The Evolution of The Earth's Surface > p. 43