Square Enix remade Final Fantasy IV: The After Years in the style of the Nintendo DS version of Final Fantasy IV, but not all of the game is included.
Final Fantasy IV: The After Years is an episodic game and each chapter has challenge dungeons that you usually have to grind a lot through to complete. For whatever reason, Square Enix removed the challenge dungeons from Final Fantasy IV: The After Years for iOS and Android. You can still get the items from those dungeons in a different way, but there is content missing in The After Years.
One thing they added though is if you have Final Fantasy IV and Final Fantasy IV: The After Years youll start the game with bonus small tails.
http://www.siliconera.com/2013/12/09/final-fantasy-iv-years-missing-content-ios-android/
They take out the one thing that played the most like a mobile game?