Zynga Casuals really should be named "Fashionable Gamers", they will only play games once it's socially accepted and peeking. After that they are done and aren't looking for the next game. Anyone with vision and paid well should know this. This is your Wii crowd, your Kinect Crowd. This isn't their hobby, it's their fad and they don't have continuous sales.
That said, one post here hit the nail to. Farmville played out like a hardcore game, the investment time becomes too much as it goes. For a fashion crowd it needed to top out to a point they can still do a little and feel good about themselves. That was their end game, as in "game over man".