If you remember the recent Nintendo game bundle offered by Humble Bundle, then you remember all of the amazing games that were offered with it. However, if you had purchased the bundle then you bought a bit more than just a few games.
It has been reported that an exploit with Citizens of Earth allows homebrew and unsolicited content to be played on the console. Those who have bought the game will still be able to redeem it but the game has been removed from the Nintendo eShop. The game now joins the ranks of games like VVVVVV which was also pulled for the same reason.
Will the game ever make a reappearance at the eShop? Who knows, but for now, if you have the game you should enjoy it. You've got yourself a collectible that not everyone can get their hands on.