State of Decay developer Undead Labs has been "thoroughly evaluating the possibility of adding co-op multiplayer" to the game and apparently determined that there isn't one. Co-op multiplayer was originally planned for the game, but ultimately scrapped as Undead Labs thought its inclusion could delay the launch of State of Decay for as much as half a year. An update on the developer's forums dashes all hopes that multiplayer might've someday made it into the Xbox Live Arcade hit.
In an update on the forums, "Undead Jeff" explains that it would take close to nine months to get multiplayer into the game. "We could 'check the box' and shoehorn multiplayer in sooner than that, but it wouldn't be a great experience, and certainly not one we'd be proud of," he explains. On top of that, pursuing multiplayer would inhibit Undead Labs' ability to support the game with further title updates and DLC.
As for future games, "Undead Jeff" says, "Multiplayer will be the absolute heart of any future State of Decay games. As many of you know, we've always had ambitious plans for the future of State of Decay, and those plans have not changed."