It happens an unfortunate lot with successful game franchises of all types. The first couple games are fantastic and innovative. Then the designers start playing it safe, either because they've run out of ideas or because they fear they will make the game too complex and thus turn off newcomers (something which, in my experience, is rarely ever the case). They don't want to threaten their cash cows. So when sales inevitably start slacking, they blame complexity rather than design stagnation, and then they decide the fix is to streamline the game to appeal to new people. This decision, of course, removes a lot of what made the franchise good to begin with and still manages to not bring in new people. They then look at the even-lower sales and say:
a) we need to go back to what worked in the first few titles (thus replicating them poorly and failing to innovate, starting the cycle again)
b) we need to streamline it more or change formats to appeal to a broader base (thus deepening the decline)
c) there isn't a market for this series anymore. Let's focus on some other titles instead (thus ending the franchise for the foreseeable future)
Examples: the Final Fantasy series, the Silent Hill series, the Resident Evil series, Simcity games, Empire Earth (oh god, Empire Earth- that last game was unplayable), Forbidden Siren, etc.
Civ has begun the cycle. From the sounds coming from Firaxis and their new-found focus on simplified, facebook-style games, I think we're at option b) for the long haul.
1 
or 0.49$ each
Oh but it has SHINY GRAPHICS!!!!! 
