SimCity's Offline Mode Has Been in Development Since August
Earlier this week, Maxis officially announced that an offline single player mode would be coming to SimCity in Update 10, but gamers have been clamoring for an offline mode ever since the game's release in March, so why did it take so long? According to Simon Fox, lead engineer on the SimCity Single Player Mode, it wasn't "as simple as flipping a switch." Because of the original vision for the game and the way the game was engineered to support that vision, several key components had to be rewritten, added, or removed in order to support a fully functioning and stable offline mode.
SimCity was originally programmed to rely on the studio's servers for critical pieces of data, such as regional information. That entire system, which had previously existed in Java, had to be recoded in C++ to produce that data locally instead of pinging the servers over the Internet. The team also had to implement "major optimizations" to ensure a smooth experience during those local calculations. In addition, trading, social features, Global Market, leaderboards, and achievements all had to be removed from the single player mode, but still be made available in multiplayer.
All in all, the engineering team responsible for the single player offline mode has been working on it since August and Fox estimates that the team will have spent over 6.5 months on it by the time it is finished. As was previously stated, the offline single player mode will be included in Update 10 at an undisclosed date.
Source: SimCity Blog