This question is going to be difficult to answer because there is no coherent standard for what "civilized" is. Rome was civilized compared to the Germanic tribes according to the Romans but who made them the arbiters of civilization? After all it could well be argued that Rome imported most of the "civilized" aspects we asociate with Rome from Greece. The way you present the question makes it sound like Rome was the one light of Civilzation in the world but if we are to make that case you would have to explain why Carthage, Greece, Egypt or Persia were not "civilized", after all they were all considered highly developed civilizations at the peak of their power. Rome did not spread civilization to these areas, it was already there, they just conquered it.

I see big problems with claiming that Rome was inherently "more civilized" than the Germanic tribes, the Gauls, the British tribes or the scandinavian lands. It was just more urbanized and complex but that does not mean it was inherently more civilized. What use did the people north of the Alps have for aqueducts? Water was abundant. Why is monarchy inherently more "civilized" than a Republic?

It sounds like what you mean to ask is why urbanization spread north during the middle ages but the way you phrased the question is going to be... challenging... to answer.