Would make the game take up more space
Somewhere on the forums I remember reading about how they will not do this because it will make the game take up more space on your phone. As it is now, all of the data is online-- so there is minimal data taking up space on your phone. If they made an offline mode, Supercell would have to make the game bigger (in data size). Supercell doesn't really want to do that. Also, as others said, it would be easier to ♥♥♥♥ because the game files would be on your phone. Having the game entirely on a server makes it much harder to ♥♥♥♥.