Here's why:

* Inverse Square Law: The intensity of light (and other forms of radiation) decreases with the square of the distance. This means that if you double the distance, the light intensity becomes four times weaker.

* Distance: The farther a planet is from the Sun, the more spread out the sunlight becomes by the time it reaches the planet's surface.

This means that planets like Neptune and Uranus receive significantly less sunlight than planets like Earth or Mars.