Here's why:

* Gravity: Planets have a stronger gravitational pull than moons. This pull keeps the moon in orbit around the planet.

* Orbital Motion: Moons follow a curved path around the planet due to the balance between their forward motion and the planet's gravitational pull.

Think of it like a ball on a string. The string represents the gravity between the planet and the moon. The ball (the moon) is constantly trying to move in a straight line, but the string (gravity) pulls it back, causing it to orbit.