Where did this Kelvin system come from?
A measuring system was needed for light and the measuring standard chosen was built around the temperature of a metal object.
Everyone has seen the color of a steel bar change as it is heated in a forge. The hotter it gets, the brighter the color of the bar. As it gets hotter, the glowing color will change from deep reds, to oranges & yellows, all the way up to white hot.
So, a system for light color measurement was developed that was based on the temperature of a metal object, and the different colors of light that were emitted as this metal got hotter.