People who look, worship (or not), work and have other things alike then to cluster together and have a sort of herd mentality.
In a place like the US, we have dramatically different groups of people all over the place. In Norway or Denmark, the differences are nonexistent to an American eye.
In Oslo, capital of Norway, 32% of the population are immigrants.
Less than 20% of Canada is a "visible minority." In the U.S., it's almost 40%. Toronto recently became a majority minority city. Washington, D.C. became a majority minority city sometime in the 1950s. In Chicago, it was sometime in the 1980s. Canada has no majority-minority provinces. Ontario is still 67% European (roughly on par with Connecticut). Our two largest states, California and Texas, are both majority-minority states.
Recency matters too. Much of Canada's diversity is very recent. American politics has in contrast been shaped by these conflicts for decades (really, from inception--we fought a whole civil war over race!).