This abbreviation of K is based on the fact that Kelvin is an absolute temperature scale where zero Kelvin (0K) is absolute zero. Celsius and Fahrenheit which have arbitrary zero points based on freezing and boiling points of water are relative and have degrees of relation whereas the Kelvin scale is based on the absolute thermodynamic temperature. By omitting the degree symbol, it reminds us that Kelvin is not a relative measurement but a unit of measurement on its own. The use of "K" instead of "°K" for Kelvin was decided as an SI convention to reserve the degree symbol for relative temperature scales.