The older games were bases on a board game.
But there was layers on top of the core rules that gave the feeling of building an empire to stand the test of time, part of that coming from the AI behavior. With 5 and even more 6, those layers were removed, with AI players giving the impression of trying to be more like human playing that board game than leaders of Civilizations. My impression is also that now we are more cities builder than Empire builder.