Joseph Heath, a professor at the University of Toronto, sees five reasons why Canada has been more successful at integrating migrants than Europe and the United States:

1. Very little illegal immigration. This helps explain the difference in attitudes with the United States but not with Western Europe, where illegal immigration is also low.
2. A political system that encourages moderation. I think this has more to do with political culture than the system. Heath argues that first-past-the-post makes it difficult for nativists to prevail. Parties need to appeal to the center. But it doesn’t stop nativists from influencing the mainstream right, as they did in the United Kingdom. To stem defections to UKIP that could split the right-wing vote and allow Labour to sneak into first place, the Conservatives felt they had to become more insular. And clearly in a two-party system, like America’s, nativists can come out on top.
3. Immigrants are part of larger nation-building project. Immigrants ended up strengthening Canadians’ sense of nationhood because, unlike the First Nations, Westerners and Quebecers, they embraced national symbols. Persuasive, but it’s hard to see how other countries could mimic this.
4. Protection of majority culture clear from the start. This is rooted in Canada’s unique history but could be a lesson to others. Heath argues that the need to appease Quebecers led to equal cultural and language protections for the English and French, as a result of which the majority felt unthreatened by newcomers.
5. Bringing people in from all over. I think this is the key. There is no “majority minority” in Canada. Heath reports that, in a typical year, no group makes up more than 15 percent of the total number of immigrants. Hence no parallel societies could emerge in Canada, like the predominantly Muslim banlieues of Paris, immigrant-heavy neighborhoods in Amsterdam and Latino districts in major cities across the United States. Their existence hinders assimilation and makes visible the threat immigrants pose to the dominant culture.

Quotas

This argues for national or regional quotas, which were formally abolished in the United States in the 1960s on the grounds that they were racist — although the visa lottery system favors African and European immigrants in order to compensate for the preponderance of Asians and Latin Americans in regular immigration channels.

Many European countries have made it harder for low-skilled migrants to qualify for permits, which has depressed immigration from Africa and the Middle East.

It may take a generation, though, for this to balance out the non-native population.

Assimilation

And diversity in itself does not encourage assimilation. That requires more of #4: protecting the majority culture.

Western Europeans have moved in this direction by conditioning citizenship on learning the national language and becoming more protective of their institutions and traits.

For Americans, who have traditionally defined themselves as a nation of immigrants, this is harder. They have a stronger civic nationalism than most, organized around the Constitution, reverence for the Founding Fathers and the flag, but anything that smells of ethnic nationalism, like making English the national language, is taboo.