Well, I'm only very familiar with the Somali situation, but there actually was a large mixed population of Italo-Somalis who grew up speaking both Italian and Somali from birth, especially in the coastal towns. This was due to a number of reasons, from cultural ones like the exogamy bias in Somali tradition to financial ones like Italian merchants marrying into nomad families to cultivate ties with the powerful Darod and Issaq clans. During the Protectorate, the Italian PCI supporters married heavily into the Somali population, with Italo-Somalis not only being important membership bases for Somali nationalist parties like the S.Y.L but later for the rise of communism in Somalia.
The problem with having an Italicized "Mestizo" class was that from the fascist period onward, Italo-Somalis by and large thought of themselves as lighter Somalis and not darker Italians. Italo-Somalis were marginalized by the colonial government, which pushed them together with their Somali relatives. Adding this to the fact that Italo-Somalis had little connection to the Catholic Church (the primary instrument of Italicization in Somalia) because they were often Muslims in the Sufi tradition or secular communists....eh, I'd have a hard time seeing them wanting to split off from the fully-native population. Somalis themselves saw the mixed Italo-Somali population as Somali, who - tellingly - were not pushed out by Barre with the Italians.