Why did congress pass a law to decrease immigration from asia?

I believe it was because of fear. At the time, many Asian (Chinese especially) people moved to America in search of a better life. Then, the Gold Rush was a thing because there was a lot of it so any worker was welcome. However, as the gold began to be depleted, foreign laborers were wanted less and less. Eventually, the fear of competition grew so strong that President Chester A. Arthur ratified the Chinese Exclusion Act in 1882, thereby denying them access into America.