That is the topic of my latest Bloomberg column, and the core of my answer is that liberalism and cooperativeness declined in the West, as WWI and the Cold War receded into historical distance (I am indebted to a much earlier conversation with Daniel Klein on these matters). But I wish to excerpt from another point of the piece:
There is another explanation for the rise in anti-liberal sentiment: immigration. Through a series of historical accidents, it was kept off the table as a major issue for many decades. The U.S. had choked off immigration in 1920, and at first the liberalization of the 1960s did not have much of a visible impact on the American population. In those early decades after the Immigration and Naturalization Act of 1965, many poor nations were so poor and unfree that it wasn’t easy to leave them.
As for Europe, in-migration was too small to make much of a political impact. For a while in the 1960s and 1970s, the bigger story was emigration, due to high taxes, from countries such as the U.K. and Sweden. The presence of the Iron Curtain also blocked some of the routes and sources that enable some migration to Western Europe today.
In a democratic society where there simply isn’t much immigration, it is much harder for nationalists and populists to use it as an issue. But today much of the West has seen high immigration for 20 years or more, giving nationalist and populist forces a major talking point. Even if most of the population is broadly pro-immigration, perhaps a core of 15 to 20 percent will not be. With that base, a movement of counterreaction can have real political impact.
Do read the whole thing.