The British colonial government of the time allowed the settlement to grow informally, primarily because of the Nubians' status as former servants of the British crown that put the colonial regime in their debt. Furthermore the Nubians, being "Detribalized Natives" had no claim on land in "Native Reserves".
After Kenyan independence in 1963, however, various forms of housing were made illegal by the government, rendering Kibera unauthorised on the basis of land tenure. Essentially, since the early 1970s landlords have rented out their property to a significantly greater number of tenants than legally permitted. Since the tenants, who are extremely poor, are unable to obtain or rent land that is "legal" according to the Kenyan government, the slum-dwellers find the rates offered to be comparatively affordable. The number of residents in Kibera has increased accordingly despite its unauthorised nature.
This process has been exacerbated because, over time, other tribes have moved into the area to rent land from the Nubian landlords. Since then, the Kikuyu have come to predominate the population and by 1974 had effectively gained control over administrative positions. This occurs primarily through political patronage, as certain owners are protected by local government officers largely based on their Kikuyu ethnicity, and changes in administrative personnel may have drastic impacts on a landlord's security.
Kibera has residents coming from all the major ethnic backgrounds with some areas being specifically dominated by one tribe (e.g., Kisumu Ndogo and Katwekera, Mashimoni which is predominantly Luo and Luhya respectively). Many are coming from rural areas due to the problems of rural underdevelopment and overpopulation. This multi-ethnic nature coupled with the tribalism of Kenyan politics has led Kibera to be the site of small ethnic conflicts throughout its near 100 year history (most recently in 2002 in which many residents' homes were attacked by arson).