Maxis' general manager Lucy Bradsaw told Polygon last week that in order to enable "SimCity" to play offline would take "a significant amount of engineering work by our team" to complete only to be contradicted by a Maxis insider saying that it wouldn't.
It seems that the Maxis insider knew what he was talking about because his claims that making an offline mode for "SimCity" wouldn't take that much work were proven true by a modder on Reddit. This was discovered after the modder found out a way to create roads outside of the region borders while keeping the changes saved and synced to the server. At that point the modder was able to turn off the server sync all together in the game, thus allowing offline mode indefinitely. However while in this offline mode, you are unable to save your game, sync your game, or utilize any of the region features. The modder closed his post with saying this was all done without much effort and considers the mod to be "very minor/easy tweaks." Keep in mind that modding the game is allowed by Maxis and confirmed by a tweet from the "SimCity" creative director Ocean Quigley.
So what exactly is Bradsaw referring to when she mentions the large amount of work that is required to make this offline mode possible? EA or Maxis have yet to make a statement since the release of the insider news or the release of this offline mod for the game. Maybe soon we can learn the truth as to why EA/Maxis wanted to require an internet connection to play this game.