Better question; why the fuck didn't he just execute Jango then and there?
Poor Kid Fisto, he ride for niggas and died for his niggas
He wanted to be able to point himself out in the super wide fighting scene on Geonosis. So they gave him a different colored lightsaber. Fun fact: it has the words "Bad Motherfucker" engraved on it.
Because Samuel L Jackson wanted to be special and wanted his kids to be able to see him in the ridiculous battle scenes, and Lucas OK'd it, and everyone didn't mind because of white guilt.