What explains that? Perhaps there's one interesting factor that may have been under-observed: Simple geography.
In the maps above, produced by Josse de Voogd and first published in Dutch newspaper de Volkskrant, Europe's recent voting has been mapped. On the left-hand side, you can see how the votes across the political spectrum were spread out. Next to it, a map shows how right-wing populism fits in.
While the maps are broad approximations, they do show how that Europe's voting habits seem remarkably consistent. For example, European cities tend to be left wing: It's easy to spot London, Paris or Berlin, little islands of red on the left-hand map. (Not everywhere follows this rule, however – in Norway and Sweden, it's clearly reversed). Right-wing populism often finds itself most at home just outside of these leftist metropolitan areas, according to Voogd's map.
More surprising still, however, is how some voting habits seem to tie into geographical surroundings. The Economist picked up on this, noting that the "flat pains of southern Sweden, East Anglia, north-eastern France, Flanders and Padania vote for right-wing populists," while the "hilly regions like Cumbria, south-west France and most of the Alps tend to stick with the mainstream parties."
The maps also show clear historical lines. Historical manufacturing regions, such as the North West of England, tend to red, as does Catalonia. The south of Holland, once known as the "Generality Lands" and economically deprived, is now a a prime place for right-wing populism, as is Flanders, the Flemish speaking north of Belgium. Alsace, once a part of Nazi Germany, has long shown support for France's National Front.
You can see the maps up close below: