Ethnically diverse countries tend to fall apart. Here is a country that clearly hasn't fallen apart. Yugoslavia is gone. Czechoslovakia is gone. The U.S. is different.
People make the decision to come to the United States.
They choose to be a part of the idea of America. The pervasive power of the American entertainment industry worldwide ensures that newcomers know a fair amount about the society they're joining. They have a reasonably clear idea of what it's about. I adore Quebec. But there is no getting around the fact that Canada is a more difficult country to maintain as a result.