This has been brought up time and again with each version of Civ.
(1) It's a problem with sales (and political correctness). The focus of the game is not about slavery. Lots of historical concepts are dropped from Civilization and are not specifically represented, so historicity is not the issue here. e.g: the concept of national borders doesn't reflect how it was imagined in the past; bio and chemical weapons are not represented; the Americans are added as a civ more for sales than history; colonialism and post-colonial nationalism is poorly done; natural disasters hardly happen; etc. heck - the map isn't even a globe, while we're at it.
(2) Slavery exists in at least 2 distinct forms - the classical-type slavery from the times of ancient Rome, and the colonial-type slavery that's associated with the mass (force) migration of Africans to the New World. So which Slavery should be represented? Even if both are represented, how do they add value to the game? As a social policy? There are slave warriors as well, so do we need special units tied to Slavery as a game mechanic? Should we dedicate an entire mechanic on configuration of labor so players can choose between tribal, castes, slaves, serfs, indentures, capitalist, communist? I'd say that's just complicating the game with little value.
(3) Some elements are already implicitly ingame - e.g. Workers are almost always considered some kind of forced labor, especially if you capture them from someone else in the early eras. In fact, this mechanic should be broken in the late eras but that's another suggestion.
(4) Why not consider the positive side of things. Rather than expect Slavery to be explicit in game, include a social idea called Civil Rights, which canvases more areas.