Could it be that the Warren Commission Report is so incredulous that mentioning Ford in connection with it would sully Ford's record by association?
Ford and Warren did interview Ruby in jail. Ford actually seemed open to taking Ruby back to Washington as Ruby requested. I think Ford was probably genuinely convinced that Oswald was the lone gunman. He was initially very interested in the reports from Mexico that Oswald had been paid off by Cuba, but when the source of this statement (Gilberto Alvarado aka "D") retracted it and changed his story, Ford was told about it, which may have made him more inclined to the lone gunman theory after that.