This made sense.
If you think of it American politics and society was established in the late 18th century and has pretty much continued evolving uninterrupted since. Yes, there was a civil war but since it was a 'civil' war count it as part of the evolution.
Whereas almost all of Western Europe had to almost start from scratch after World War II. From the rubbles, Western Europeans adopted democracy, chucking monarchies and, perhaps, redoing social norms.
And the latest, youngest, are what was referred to as Eastern Europe, still experiencing teething pains.