He was great, but the only unanimous selection? Over Babe, Ted Williams, Lou Gehrig, Hank Aaron, Willie Mays, Ty Cobb, Mickey Mantle, Jackie Robinson,......... What was wrong with those guys?
But you could make the case the Riviera is head and shoulder above any other relief pitch in MLB history, with the possible exception of Dennis Eckersley.
The reason is simple: there were a bunch of writers on the Baseball Writers Association of America (BBWAA) that wouldn't vote in a first-time eligible nominee "out of principle." It's only recently when the BBWAA started to purge those old-time voters that it made it possible for Mariano Rivera to get the unanimous nod.
It is difficult to compare because Rivera played on the best franchise in the league during his career but they were also successful because he was so dominant. In any case, if ballots had been scrutinized like they are now you would have had a lot more unanimous picks.
Dumb voters. Then it became a “tradition” one of baseball’s many unwritten rules was “if Babe wasn’t unanimous nobody should be” so there was always a cadre of 3 or 4 voters that “made sure” nobody went unanimous. Apparently they all died.