It's a tricky situation. I know they fell out will Bill over some silly contracts issue, so he's out. I guess Vinny Appice was always more Rionnie's man than Tony's or Geezer's, so maybe he wasn't considered. And if you bring in Tommy Clufetos or whoever, it becomes perhaps too much of an "Ozzy album".
I assume it was Rick Rubin's idea to get Brad Wilk.