In their main article the authors don't speculate on why WEIRD folks act so differently, but when pressed by comments they suggest:
[Consider] the relative strangeness, in a broad global and historical context, of modern middle-and upper-class American beliefs, values, cultural models, and practices vis-a`-vis childrearing. … These practices impact cognitive, linguistic, and motor development, including long-term cognitive outcomes. … We speculate that in the context of mobile, meritocratic societies, … cultural evolutionary processes rooted in our evolved tendencies to imitate successful and prestigious individuals will favor the spread of child-rearing traits that speed up and enhance the development of those particular cognitive and social skills that eventually translate into social and economic success in these populations. This kind of cultural evolutionary process may be part of what is driving the dramatic increases in IQ observed in many industrialized nations over the last century, along with increases in biases toward analytical reasoning and individualism. It would also explain the obsession with active instruction of all kinds shown by middle- and upper-class Americans.
It sure seems that these Canadian authors are suggesting that the US (which on a world scale is almost like Canada) is different mainly because the US is better: stronger US competition has more quickly selected for kid-raising norms that make more successful kids, and work norms that are more productive. Seems a remarkably self-centered interpretation for an article claiming that US psychologists are too self-centered. Doesn't make it wrong of course, but it is noteworthy that they didn't even notice its self-centeredness.