Why does water boil at 100°C only at sea level?

Water boils at 100°C only at sea level because this temperature is the definition of water's boiling point at standard atmospheric pressure. "Sea level" is the globally accepted baseline used to measure and define this standard pressure, which is 1 atmosphere (atm) or 101.325 kilopascals (kPa). 

The 100°C value is fundamentally tied to this specific pressure measurement. If your altitude is different, the pressure is different, and therefore the boiling point will also be different. At any altitude above sea level, the pressure is lower, so the boiling point is below 100°C.