That did happen eventually but for the pre mercantile period we're talking about it's either due to being a frontier colony for the romans or afterward quite decentralized in power from a regional perspective. Less central control meant smaller scale public works and lesser development of urban centres and the pre existing ones were beginning to fall to shambles due to lack of maintenance from a building and infrastructure perspective and disease from a human perspective leading to urban centres being generally abandoned fornthe countryside. Urban life in Rome and similar cities was pretty ass and without a strong central government making sure slaves kept the food coming in and maintaining the structure necessary to keep an urban area going, it was basically leave or die. This led to central and western Europe having a more disperse population with less centralized control and greater regional power than before. Urban centres weren't really built by the romans in Northern central and wesrern Europe and in the middle ages there wasn't really much material reason to try to build new ones of equal scale and power dynamics were often a bit too chaotic to do so.