This answer can take a book to answer, and does. But, long story short, it has to do with the introduction of sedentizing technology and institutions from Western Europe. The advent of crops, farming methods, colonists (particularly during the Carolingian era) and technology resulted in more people in central Europe settling down to engage in farming; and where farms form, communities tend to gather, which in turn become towns and cities.

The decline in the nomadic lifestyle of many of the germanic tribes also goes a long way in explaining why people in central europe formed cities. Basically, Eastern Europe ran out of people to send west. With each successive wave of migrants, the previous inhabitants had been displaced, with no one being able to stick around long enough to form cities. It was only around the 6th-7th century that migration became more sedated; to the extent that it was no longer necessary for inhabitants of central europe to flee across the Rhine every few decades in fear of a new eastern menace.

Any additional details would be appreciated; feels like there is a tranche of details being seeped under the rug with that explanation.