Simple. Grand juries can only act on the information presented them! No evidence presented = no other culprits!
Which is why I included petite juries, which means that there were two completely different defense attorneys who had an opportunity to "muddy the waters" and blame someone else and at least mitigate damage to their clients; and, more important, ANY info their clients could have provided would have certainly helped in a deal. Maybe McV. didn't want one, but Nichols sure did. Yet with all that riding on it, no defense atty. ever managed to introduce one single piece of evidence into the trial record that would take even some of the blame of McV. or N.