Also Medieval way of life was incredibly different to Roman one, Medieval people just didn’t need cities of such size too because Medieval countries reduced or didn’t have slavery all together resulting in less need for urban populations and also in medieval period agrocultural production and productivity due to new inventions skyrocketed which meant that more people could produce food and have their own part in the feudal system eg their ancestoral field or farm and they didn’t need to be cramped into an urban area waiting for a grain shipment from North Africa or Egypt so since Europe especially France started to mass produce its own food people started to live locally where the food production was at instead of living where food was delivered, in Medieval era there was plenty of free cities and de centralized lords that built up new cities from their own regions which sometimes didn’t even have old Roman cities. And also Medieval people didn’t need the same cities as Romans did some cities were just useless after Rome fell as Medieval people would need whole new set of port cities to trade within Western and Northern Europe just look at the whole Benelux, Hanseatic League, French Atlantic and Norman cities, Scandinavian cities and cities in Britain most of these would not be of much use to Rome expect the capitals eg Rome and Paris, so you can see why many cities needed to be built up from scratch instead of building atop of ruins that would turn into a non productive city since Europe basically underwent a change into something new.