Short answer: fatigue.

Longer answer: "Earlier this week we examined why Zynga is shedding users, causing its stock price to almost halve from its December IPO level. Zynga rose to prominence on the back of a farming simulation social game called FarmVille. Its success spawned a series of other 'Ville' games, such as CityVille, CastleVille, FishVille and PetVille. The theme of these games is to manage virtual stuff, with the help of virtual currency that you buy within Facebook (using real currency). The Ville series became tremendously popular, despite being shallow and irritating to many Facebook users - whose news feeds became clogged with virtual sheep, cartoon fish and the like. Mercifully, for most of us, the Ville dynasty appears to be dying out. In this post we explore why."

Longest answer: read the article.

good riddance