It is because of the cultural make up of the country. For many years there has been a constant stream of immigration into the US from all over the world. There are large communities now established who follow their own religions and their unique ways of life. The term 'melting pot' merely means that in the course of everyday life all these communities interact with one another and pick up ideas from each other. Sometimes these ideas are taken up. A good example of this is in the culinary field where recipes that originate from other cultures are utilised in the home.
America is called a melting pot because it is made up of people from many different countries .