Most recently, due to ordoliberalism and coat-according-to-cloth morality guiding economic policy rather than money printer go brrr.

Longer term: cultural and language divisions despite attempts at creating a common market, not running the global reserve currency/military hegemony, social democracies encouraging work-life balance over cutthroat careerism, demographic issues, not getting a boost from being the only consumer economy not to be leveled in WW2, etc.