Why was the United States called a "melting pot"?

The United States is a huge nation, the fourth largest in the world. Within its borders, there are vast regional differences in climate, geography, and historical experience. Each section of the United States is often thought to have its own customs and attitudes, and stereotypes have developed about the people of each region.
   The United States has long been known as a melting pot because so many people from all over the world make up its population. The first immigrants in American history came from England and the Netherlands. Immigrants from many other countries then began arriving and are still coming. The greatest numbers came in the years 1880 to 1914. Between 1820 and 1973, the United States admitted over 46 million immigrants. Most came from Europe, but many also came from Latin America, Asia, Africa, Australia and Canada.