London has a large private rented sector and plenty of accidental and overseas landlords, many of whom own a single flat in a converted Victorian house or a new-build block. The rules are heavier than in most of the country.
Letting agents must belong to a redress scheme and hold client money protection, many boroughs run additional or selective licensing schemes, and the Renters' Rights Act has changed how tenancies start and end.
Landlords feel that weight and are more willing to pay for full management than for a tenant-find service.
The borough matters too: Islington and Camden landlords often own period conversions, Tower Hamlets has many new-build flats, and outer boroughs like Richmond and Bromley have more family houses.