The same reason Lou Marino has never been indicted - there was no other evidence to corroborate Nick's testimony. Michael Spilotro's daughter testified that Marcello was the one who called the Spilotro household on the day of the murder and in addition to that, the feds also had as evidence the wiretaps disclosing the "hush money" which Marcello was paying to Nick, presumably to keep him quiet about his involvement in the Spilotro murders.

No such evidence exits for DiFronzo or Marino, the only two survivors out of the remainder of the alleged killers.