Added each civ with a unique ability is the step forward. That requires consideration of each civ from scratch. Adding voice actors for the civs is a new addition. That requires compiling the resources for civs where they are starting from scratch. Having full leader screens that loads separately from the game as a whole requires recreating them from scratch. Yes, they could have kept the old civs and their artwork and just gave them traits by leader (a direct copy and paste from Civ4). In that case, they could add new civs like Polynesia without having to recreate the old ones. But they didn't do that. All artwork in the game was created from scratch, which takes time, effort, and money.