LogFAQs > #979513188

Because Sonny Corleone wasn't actually human. He was a puppet that was hand-crafted by Vito and which was brought to life by the Blue Fairy to be his son (as payment for a favor she owed him), but as per the laws of La Cosa Nostra, he would not be allowed to take his father's place as head of the family until he became a real boy. Unfortunately, Barzini had him killed before he could achieve that, and the Fairy had asked only two favors of the Don in her life (the second of which had already been called in, when Fredo pestered his father to have her bring a banana daquiri to life), so Vito couldn't call on her again to bring Sonny back.