Here's why:

* Gravitational force is incredibly weak: It's the force that governs the attraction between objects with mass. While it's responsible for keeping us on the ground and planets in orbit, its strength is minuscule compared to other forces.

* Short-range forces: The strong and weak nuclear forces only act over extremely short distances, within the nucleus of atoms. The electromagnetic force acts over much longer distances, but still pales in comparison to gravity's reach.

* Gravity's cumulative effect: While weak on an individual particle level, gravity's influence becomes significant over vast distances and large masses. This is why it governs the motion of stars and galaxies.

While gravity is the weakest of the fundamental forces, it's also the most influential over large scales.