Pretty much what the title says: It's my understanding that the Romans had by and large abandoned London by around the 5th century CE. Within a short time after (maybe about 50 years or so) the Anglo-Saxons started their settlement, Lundenwic, but instead of using existing Roman buildings and infrastructure they built their city right next to the Roman settlement, outside the city walls. All the texts I've read about the issue gloss over the reasons. Maybe I'm having the wrong impression of how re-usable the Roman site was, but it always struck me as odd, especially since 300 years later they eventually did move the city back into the walls. Is there a consensus amongst historians?
One of the major architectural elements of Alfred's reoccupation of London in the late 9th Century is a significant remapping and reconstruction of the road network, in particular the clearing of areas around the river. What this suggests is that by the time of significant early Anglo-Saxon occupation, the Roman docks may have been in significant disrepair, or simply not suitable for the more shallow-draft vessels that were typical to the Channel region at the time. The abandonment of the cities was not an overnight event, but it's likely that as the Roman Imperial infrastructure declined, it simply became too difficult to marshall the appropriate resources needed to keep the buildings of Londinium in good repair, and Lundenwic became a more convenient and viable site for trade rather than among the crumbling ruins of the old city. Archaeological and numismatic evidence has shown that the old London site was still occupied during this period, but that it was simply overshadowed by its immediate neighbour. By the 9th Century, Mercia and Wessex are able to more easily marshal the manpower and material necessary to make the repair and reoccupation of the Roman site viable and worthwhile.