I was listening to a breakdown of this study on a New York Times podcast. It has to do with huge cultural differences between how Europeans and Americans interact with smartphones in cars, particularly because most cars in the United States are automatic and most cars in Europe are stick shifts, meaning that it’s very difficult for Europeans to screw around with her phones while they’re driving. Driving a car with a manual transmission requires both hands, meaning drivers, don’t have a free hand to fiddle with their phones.
Another part of the explanation for the difference between the United States and Europe in this regard is that suburban United States cities are designed in the auto age and designed very much around cars with a complete disregard for pedestrian safety, particularly at night. American pedestrians in these cities have to walk much farther and around much larger and more dangerous roads to get to their destinations, while having access to poor or even nonexistent transit networks. 
edit: one other data point they mentioned was the homeless, and while that population was rising in 2009, it sharply began to rise in 2016. these are people who are the most vulnerable in our society already, who often dwell near dangerous roads, highway overpasses, etc., and especially at night. Homeless people account for a significant portion of the increase in pedestrian fatalities in certain regions.