Danes still own as many cars as everyone else, it's not really an outlier and the number of cars per capita is growing.
https://ec.europa.eu/eurostat/databrowser/view/road_eqs_carh...
https://ec.europa.eu/eurostat/databrowser/view/road_eqs_carh...
[1]: 0.540 for Denmark vs 0.868 for the US (https://en.m.wikipedia.org/wiki/List_of_countries_by_vehicle...)
If you start correcting for other variables, you'll have to look at demographics, size and flatness of the country, prices, public transport availability, population distribution between rural and urban areas etc. etc.
Relatively poor Romania has a lower number of cars per capita and if you live in Bucharest like me, you'd believe everyone owns a car (half being Dacias), nobody rides a bicycle.