The thing about founding colonies is that it's not really a basic function of state. It takes a particular set of circumstances and incentives so that a state, or people acting in the name of a state or monarch, to go and conquer or colonize another place.

I wrote about Italian colonization, or lack there of, in this older answer as well as this one; although both are more about the Americas than Africa, you might nonetheless be interested as they touch upon more general themes impeding the Italian states from colonizing.

I'd also point out that the Republic of Venice did indeed have an extensive colonial empire in the eastern Mediterranean, encompassing all of Istria and Dalmatia as well as the islands of Crete, Corfu, Negropont, and Cyprus. However, these colonies were useful stopping points along the trade routes to Alexandria and Istanbul, as well as bases from which to project power in the Adriatic and Aegean seas. Venetian colonization and expansion doctrine never really favored the occupation or seizure of large urban settlements outside of their immediate neighborhood; preferring instead to hold smaller, more manageable outposts. In larger cities, like Alexandria and Constantinople, Venetians preferred to stipulate contracts allowing them to live near the existing population.

Although the Venetian presence in Egypt was certainly important, on the whole North Africa was not at the terminus of major land-based trade routes as the cities in the eastern mediterranean were. Additionally, Venetian presence in the western mediterranean was mostly geared towards taking finished goods to consumers, rather than the collection of raw materials or goods for resale as in the East. So you can imagine how the Venetians were generally disinterested in North Africa.

Lastly, I'd point out that Venetians only really established colonies in those regions that were integral part of the Venetian macro-economy, where merchants and aristocrats had tight economic, familial, and social ties to Venice. I wrote a rather long-winded, but nonetheless colorful and descriptive narration of the mechanism of the Republic's territorial expansion here which you might be interested in.