You might want to read the Court's opinion which was rendered in response to several cases none of which was a california case. As typically happens, the Court remanded those cases for further proceedings consistent with the Court's opinion. In light of the Court's opinion, the State of California could anticipate a large number of appellate hearings based on that opinion thus tying up significant State resources. Further, the necessity for a revision in california criminal statutes and the challenges to their compatibility with the Court's opinion would have simply achieved what the commutation of Manson's sentence achieved.