After the 2008 financial crisis (when the article claims the divergence started). The US and the EU chose different paths. The US chose bailouts and the EU went with austerity. Both were very unpopular, but one of them was also stupid. Helping the “wrong” people, it turns out was a better choice than trying to actively hurt all the people.
I’m sure there is a lot of other factors at play but EU governments getting religion about belt tightening and small-government in ‘09-‘12 was phenomenally stupid.