I'm guessing the key is that Algeria is so close to France, only separated by the Mediterranean, and maybe there was a history of French-speakers settling along the African coast that I don't know about. And I wonder if Italy might have planned to do something similar with Ethiopia if they'd been successful there (though at that time it would have taken much longer to go to Ethiopia from Italy than to get to Algeria from France).
I'm not familiar enough to answer about French Algeria, but I can answer in Italy's case.
Italy was mostly interested in Tunisia(which had a large community of Italians, but eventually became a French colony in the 1880's) and especially in Libya.
Libya was considered to be the "Fourth Shore" of Italy and vital for Italian control of the Mediterranean. Considerable resources were invested in infrastructure, and Italians were encouraged to settle, reaching around 10% of the population by 1939(100,000 people) and a goal of 500,000 people by the 1960's. Libya was also integrated into metropolitan Italy in 1939, in the same way that France integrated Algeria.
Italy's colonies in Somalia and Eritrea also had a significant settler population, but their occupation of Ethiopia only lasted from 1936-1941. It was administered as a colony but I am not aware of any plans for it to be integrated into Italy itself. Italian East Africa had a much higher indigenous population than Libya and it may not have been as feasible.
World War II resulted in the loss of Italy's colonies and therefore put a halt to their settlement in Africa.