Read the following passage carefully and then answer the questions below

Traditionally the United States has been described as a melting pot, a place where the previous identities of each immigrant group are melted down to create an intergrated, uniform society. Since the 1960s, many Americans have rejected the melting pot metaphor in favour of the image of the mosaic, a picture created by assembling many small stones or tiles. In a mosaic, each peace retains its own distinctive identity, while contributing to a larger design. Advocates of the mosaic metaphor assert that it better represents the diverse multicultural society of the United States. Today, may Americans value their immigrant heritage as an important part of their identity. More recent immigrant groups from Asia, such as Vietnamese Americans and Korean Americans, have established communities alongside those populated by the descendants of European immigrants, such as French Americans, German Americans, Irish Americans and Italian Americans. 

Why has the United States often been described as a melting pot?