History I guess. Germanic people lived next to the Roman Empire and were able to use the Roman achievements for themselves. Hell, Germanic tribes managed to defeat the Roman Empire in the end, which is a massive achievement on its own. Western Roman culture fused with German culture and became the most important traits of the unified post-Classic European culture. In time, Catholic Christianity and Roman legacy became the defying features of Germanic culture to the point men like Charlemagne saw themselves as inheritors of Rome. Germanic people expanded eastwards and encountered less sophisticated Slavs, which were at the similar level as Germanics before Germans fused with Rome. What followed was centuries of fighting (Wendish Crusades, Ostsiedlung, Drang nach Osten) which ultimately culminated in German victory as Slavs were pushed eastwards.
However, history is never simply black and white. Countries like Poland or Czechia were never far behind the West. Especially Czechia, which for some time was one of the wealthiest and most important lands of the Holy Roman Empire. When Otto the Third coroneted Boleslaw the Brave as the King of Poland he proclaimed Boleslaw as a friend of Rome and supposedly said ''there are no easier friends than Germans and Slavs'.
