There are a number of factors. There are great modders for Civ5 and the game is theoretically very moddable and there are some notable mods even if the platform is so slow to work with. I think another factor is that the AI in Civ4 intuitively felt like it could be improved rapidly. The AI in civ5 needs so much work. The AI is playable but 1UPT means the workload on the coding is ten times what it was in Civ4 (I mean that).