If I recall (don't quote me) is that Civ 5 loves single threaded performance. Since it is a turn based game, a lot of things are sequential and depend on what happened before. Thus it really hammers a single thread used for the ai and such.
Real Time Strategies do not need as much single threaded performance since if program correctly each unit can have its own AI and such and thus can saturate multiple cpu cores. With RTS what is the big determining factor is that time is always moving, unlike a turn based game where your actions are static and can not move until you see what city A does to city B, or what AI 1 does to AI 2 and AI2 does to the player.