Because a Uchiha knows the Uchiha district and every member.Why didn't they get them to deal with uchiha. Why did hurizen leave the fate of the village to a 7 year old.
Because one guy is the best choice for a silent serie of murders.
Because banning one guy is better than banning one of the best clans of the village.
Because Uzumaki are dispatched around the world.
Because Hiruzen doesn't want an entire clan to know that he can decide to genocide a clan for the village.
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