As Inside Social Games notes, it seems like an odd move for Zynga, which has lately been trying to decrease its dependence on mega-network Facebook. Then again, it's not the first time Zynga has decided to pull its games from a social network -- see the company's canceled relationship with relatively minor network Tagged.
While having access to MSN's millions of users surely didn't hurt Farmville, Zynga likely decided their resources were better spent focusing on other markets, such as Japanese mobile phone networks, for instance. As we saw with Zynga's canceled Facebook game Street Racing, sometimes the opportunity cost of the status quo outweighs the benefit of leaving things as they are.
We've reached out to Zynga for a comment on the move and will let you know if and when we hear back.