The united states is sometimes called a "melting pot" because

Because it is a nation of immigrants

People say that it's a melting pot because America is a country of immigrants that have blended into one culture. For example, let's think of America as a melting pot, as immigrants came in from around the world, there cultures mixed in with them. Now, it's called a melting pot because cultures melt into American society. "Melting Pot", is a metaphor for a diverse society that blends together as one. Like America, so that's why America is sometimes referred to "The Melting Pot of The World's Nations".