I think it’s just a “how likely do people assume the populace is to move against its own government” thing. The French and the former Soviet republics should be obvious, but the Spanish civil war ended way more recently than people think and Italy had a bunch of covid protests. Everywhere else did too, but because Italy was further along it hit the news earlier and became more memorable.

Since it’s modeled on stereotypes, it’s either something loose like that or just the “hot blooded southwestern Europe” idea.

If people knew about that time the Dutch cannibalized an unfit ruler, they’d be considered plenty revolutionary, and if people realized that the Russian peasants were living under serfdom until the 1800s, they’d probably consider them less revolutionary.