Why not use degrees when talking about Kelvins? Well to answer that, we need to make another question… what is a degree anyway?
A degree is a division of a pattern, for example when talking about angles, we measure them in degrees (that is, when we are not using radians), the full circumference of a circle is said to be 360° (degrees), an arbitrary convention that we have been using since the Babylonians, and what they did is that they divide the circle into 360 sections and each of those sections is what we call a degree. So a degree when talking about an angle is ⅓60 of the circumference of a circle. The logic is the same when measuring temperature, and the pattern here is just as relative as the used by the Babylonians.
Since heat changes the state of matter, and temperature tells us how much heated something is, we can make a pattern that can start when there is little heat and ends when there is much heat. But ‘much’ and ‘little’ are too subjective to use for measuring purposes. Lets say that the start of the pattern will be assigned when water freezes and the end will be assigned when water boils. If that pattern is divided into 100 sections, or degrees, assigning 0° when water freezes, and 100° when water boils, and each one of those divisions is what we know as the measuring unit of Celsius degree. It is the same for Fahrenheit degrees, but that same pattern divided in 180 sections or degrees and assigned 32° when water freezes and 212° F when water boils.
There is no pattern like this when talking about Kelvins. It uses that absolute reference of when molecules cease to vibrate, this is when they have no heat(heat makes molecules vibrate), and is stated to be as 0 K (Zero Kelvin, absolute zero, -272°C or -459.67°F), every other temperature takes it from there, those temperatures are a reference to that temperature that is measured when matter is at absolute zero. So there is no pattern when talking about kelvins, therefore there is no need to use degrees (though one Kelvin unit is the same as a one Celsius degree) when talking about Kelvin units.