Here's why:

* Newton's Law of Universal Gravitation: This law states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

* Sun's Massive Size: The Sun is incredibly massive, which means it has a very strong gravitational pull.

* Earth's Orbit: Earth's orbit around the Sun is a balance between its forward motion (inertia) and the Sun's gravitational pull. If gravity were weaker, Earth would fly off into space; if it were stronger, Earth would spiral into the Sun.

While other forces exist (like the gravitational pull of other planets), the Sun's gravitational pull is overwhelmingly the dominant force responsible for Earth's orbit.