Think of it this way, if you take a wire with insulation and start to heat it up what happens to the insulation? It melts off. The same type of thing holds true with electronics, the silicone, germanium and other materials used in making the internal items of the electronic components starts to break down as it gets hotter as well. So if you dont have the heat dissapation then your component is going to fail prematurely, hence derate for safety and longevity.