Here's why:

* Kepler's Laws of Planetary Motion: These laws describe how planets move around the Sun. The second law states that a planet sweeps out equal areas in equal times. This means that when a planet is farther from the Sun, it moves slower to cover the same area as when it's closer.

* Gravity: The Sun's gravity weakens with distance. When a planet is farther away, the gravitational pull is less strong, causing the planet to move more slowly.

* Elliptical Orbits: While planets' orbits are not perfect circles, they are more elliptical when they are farther from the Sun. This is because the gravitational force is not constant throughout the orbit.

In summary, a planet's orbit is influenced by the Sun's gravity, and the farther it is, the weaker the gravitational pull and the slower and more elliptical the orbit becomes.