What are the main four reasons the Chinese immigrants to America?

Here are a few reasons some Chinese immigrants came to the US:

1 The California Gold Rush began in 1848 as gold was discovered by James Marshall at Sutter's timber mill.

2 The Taiping Rebellion (1850 - 1864) against the autocratic Qing dynasty led to the deaths of at least 20 million Chinese peasants.

3 The 1852 Flooding of the Huang He river in the Shandong Peninsula led to disastrous crop failures, famine and disease in southern China.

4 The Northern Chinese Famine of 1876 - 79 devastated China and up to 13 million people are estimated to have died of starvation and disease.