We'll almost certainly see them as being among the first DLC, but I can understand the reasoning behind not including them at launch. They're far less numerous than feudal lords, and will correspondingly have a much lower number of players wanting them as their first choice, and at the same time they require a completely different set of comprehensive mechanics to properly simulate their internal political structure and economy, and the developers likely don't want a repeat of CK2's republics being almost completely unmoddable once implemented. I will gladly wait a while longer if it means seeing the Venetian cursus honorum implemented properly, and Guelph/Ghibelline conflict in the Italian city-states.