Probably because there's a cathedral in there already. Inside the walled area is the cathedral church of St Paul, overseen by Bishop Mellitus, as part of a drive to Christianize the Anglo-Saxons by Pope Gregory I (Augustine in Kent, Mellitus in Essex, Palinus in Northumbria, Felix in East Anglia, Birinus in the Thames Valley). Mellitus was lucky in that he convinced Aethelberht of Kent, to baptise his nephew, Saeberht, who then allowed Mellitus to establish the bishopric in 604AD. Aethelberht had married a Christian Frank called Bertha, so there's a fair amount of Christian influence washing around the family.

Dominic Perring says that only the church was encouraged to establish itself within the walls but doesn't give a footnote, but my guess is that he's going off on charters which would have been explicit allowances for certain activities within certain areas. The Roman mission to England came from Italy and Gaul, where the concept of the Cathedral is very important - Bishops have governing and social/political importance in a landscape that is in decay - so the establishment of cathedrals is very important, as it's the civitates for the post-Roman world.

There may be other two reasons which I'm less certain on, but they're reasonable enough as supporting evidence. The first is at least in the immediate post-Roman time, the area would have been difficult to substantially defend without a lot of people, and while there is an estimate of perhaps 100 houses in the area in the 3rd century, they had mostly gone by the 4th. The second, in line with the establishment of the cathedral, might have been that the area is seen as an elite location - administrative and not economic, so Lundenwic moves just outside the area because it you don't do trade and industry in those areas.