I can only speak about the Italian colonies.Basically for three reasons:

First, the loss of control over the colonies was sudden and forced by defeat in the war, with no way to maintain contact with the newly independent states that had emerged from the colonies. In many cases what would have been the role of Italian (administrative and commercial language) was taken over by English in the immediate postwar period.

Second, European languages play the role of lingua franca in Africa, in states comprising many ethnic groups each with its own language not understood by the others. For example, in Nigeria there are more than 500 African languages and everyone speaks English to understand each other from one end of the country to the other. But in the Italian colonies (Libya, Eritrea and Somalia) this role was already played by Arabic.

Third, the individual histories of the various post-colonial states. In Libya in 1969 Qaddafi came to power and carried out a policy of fierce Arab nationalism. Over 30,000 Italians who remained in independent Libya were forced to leave the country, and the use of Italian was suppressed as unpatriotic. In Somalia (entrusted to Italy until the 1960s) Italian was widespread in city centers and among the upper classes, but the collapse of state institutions made it impossible to maintain. In Eritrea, the most loyal of the colonies, Italian is still used for commerce, but little and only in the capital Asmara. In short, don't rely on it if you ever want to go to Eritrea.

All this talk is not about Ethiopia, which was under Italian control for only five years and already had its own state institutions with its own language.