Sheepy's massive production change was really just the straw that broke the camels back for a lot of us. Most of us were already tired of the game as it really hasn't grown more dynamic or interesting with age and were barely active as it was. After Sheepy made those changes it essentially forced all of us to rethink our builds and edit all our cities and most of us just didn't feel like going through all that effort for a game we've come to grow bored of.
Personally I voted to stay, and the alliance vote was for the most part an even split, however none of those who wanted to stay were really active enough to lead the alliance. Adding to that many of the people who voted to stay were already pretty apathetic towards the game and only voted to stay out of hopes of keeping the community alive in the game, not because they liked the game itself.
I often find myself in the minority in Mensa as I'm the most likely to defend Sheepy, but the game has gotten stagnant and he hasn't done much to change that. His latest change to production did fix the inflation problem in that it raised prices a bit, but over time the markets will adjust and the inflation will come back so it really just kicked the can further down the road and created a headache for everyone. Basically rather than add to the game to make it more interesting, Sheepy just tweaks the formulas and calculations. This shakes things up a bit, but people react quickly and it doesn't change the overall game play at all. Now had the game ever added the ability to fight over actual territories on the map or created a more dynamic economic system with various consumer goods that provide certain bonuses I think a lot of us would have been fine staying.
Bottom Line: After two and a half years we just got tired of wars that don't mean anything, and the shallow two dimensional war and economic modules.