That's because, according to creator Dean Hall, the dev team "had the chance to go from making a game that was just the mod improved slightly, packaged simply, and sold -- to actually redeveloping the engine and making the game the way [the team] dreamed it could be." The new plan is for a closed test of 500-1000 participants to happen in the near future, after which the internal release date will be adjusted as the team feels necessary.
As the team has taken extra time to polish things up, a great deal has changed. Items have more complexity now -- picking up a shirt from a player who suffered from cholera may well get you infected, and shooting a player in the head in an attempt to get his night vision goggles may result in damage to the goggles themselves. The UI is being streamlined and simplified to be "straightforward and functional just like in Minecraft." Map redesign and refinement is being slowed down by the continued imprisonment of the team's lead architect.