Why is the degree mark not used in Fahrenheit?

Originally the unit was referred to as a "degree absolute" and written oK. In the late 1960s it was decided to specify precisely what scale was being referred to (there's another absolute temperature scale called the Rankine scale which is based on the Fahrenheit degree). So, the unit was changed from "degree absolute" to "kelvin" and the abbreviation was changed to drop the degree mark. This brought the fundamental SI temperature unit in line with the other fundamental SI units.