Well, its not. Technically, the sun is orbiting the center of our galaxy like we orbit said Sun. However, if you mean ‘why do we orbit the sun instead of the sun orbiting us or both of us orbiting over an invisible axis?’ then it’s because the sun is SO MASSIVE! All of the planets masses combined would have very, very little gravitational effect on the sun, therefore, the sun pulls us inward. The only thing preventing us from falling directly in is the fact that Earth is moving at about 67,000 miles per hour!
Think of it like this:
Imagine you had a sheet hanging taunt in the air. if you put two marbles on the sheet, at different ends, and pushed them at the same speed and at equal yet opposite angles. Once they get to the center, they will “orbit” around an invisible axis until they lose speed and collide in the middle. Now that would be a binary solar system, or like two planets, or a planet and a moon that’s almost the same size (like Pluto and Charon).To see why the sun has control, replace one of the tiny marbles with a bowling ball. Strike that, make in a cannon ball. Immediately, you will notice that the marble moves around the cannonball just fine, yet the cannonball won’t move at all. If you tried to have Sol orbit us, we would either end up getting sucked into the Earth or flung far out of orbit, and everything will go back to the way it was (minus one Earth).
Tell you what, if you’d like to explore this further, there is a website I found a while back that’s pretty cool. You may have heard of it. Go to this link, My Solar System, and click download (don’t worry, it’s perfectly safe). It will take you to a page where you can create your own solar system, and make your own masses, planets, etc. and see how they work, with angles and velocity and such. Let me know what you think!