As the ejected shell of mostly unburned material gradually expands and cools, it emits less light in accordance with its change in size and temperature.
Decreasing rates of nuclear fusion within the core of the remaining material cause a gradual dimming.
Radioactive decay of unstable heavy nuclei created in the explosion releases a gradually decreasing amount of energy.
Material raining back down onto the star after its explosion gradually covers the cooling remnant.
Changes in the rate of expansion of the stellar remnant create a gradual decline in brightness.