Simple enough. In gun free, "multicultural" zones like Belgium, France, or Germany muslims have infiltrated the fabric of the nation with their sheer numbers, and since they are the only ones with the weapons, they will impose their will on the local populace, through threats and outright terror.
An attack in America of this scale would create a huge armed backlash, and they know it. Remember, if the cops can't protect us from them, the cops can't protect them from us either.