Degree symbol used on Celsius and on f but not on k?

Answer 1

Because when expressing absolute temperature using the Kelvin scale, the unit

of temperature is not the "degree", it's the "Kelvin".

The "Kelvin" is a unit of temperature equal to one Celsius degree. There's no such thing

as a "Kelvin degree".

Answer 2

The degree symbol is used in both Celsius (°C) and Fahrenheit (°F) to indicate temperature measurements. However, the Kelvin scale (K) does not use the degree symbol because it is an absolute temperature scale that starts at 0 Kelvin, where there is no negative temperature.