why is the sun the primary gravitational force in the solar system?

It is not correct to say that the Sun is a force. Gravity is a force of attraction between any two bodies that have mass. The gravitational attraction to the Sun is far greater than the attraction to any other body in the solar system. To explain this, consider the equation for the force of gravitational attraction between two objects:
F = (Gm_1m_2)/r^2
Here F is the force of gravitational attraction. Note that this force acts equally on both bodies. We often think of Earth and planets being attracted by the Sun, but force of gravity on the Sun from the Earth is equal to the force on the Earth from the Sun. G is the gravitational constant, a number that is known and can be looked up in a table. The variables on which the force depends are m_1 , m_2 , and r , the masses of the two bodies and the distance between them.
The Sun’s mass of about 2 X 10^30 kg is by far the greatest mass of any object in the solar system. It is about a thousand times as great as the mass of Jupiter, the next most massive object. Thus, the force of attraction between the sun and most other objects is generally greater than the force between other pairs of objects.
Less massive objects that are close together, so that r is small, may still be strongly attracted. This is why the moon and the satellites of other planets revolve around their respective planets. For these satellites, the closeness of their planets leads to a stronger attraction to the planet than to the distant Sun. Still, the Sun’s mass attracts all of the planets, asteroids, and comets in the solar system so strongly that all of them revolve around the Sun, and the entire solar system is thus organized around the Sun.