Here's why:

* Kepler's Third Law: This law states that the square of a planet's orbital period (the time it takes to complete one orbit) is proportional to the cube of its average distance from the Sun.

* Gravity and Distance: The force of gravity between the Sun and a planet weakens as the distance between them increases. This means that a planet further away from the Sun experiences less gravitational pull, resulting in a slower orbital speed.

* Longer Path: A planet farther away from the Sun has to travel a larger circumference to complete one orbit. This longer path, combined with slower speed, results in a longer orbital period.

In simpler terms: Imagine a planet close to the Sun like Mercury. It feels a strong pull from the Sun, so it zips around quickly. Now imagine Jupiter, much farther out. It feels a weaker pull, so it moves slower and takes a longer time to complete one orbit.