I think Mean Joe makes sense because he is, quite plainly, a Steelers and NFL legend. The players around him on those 70s dynasty teams may have contributed to his success, and some of them may have done great things in their own right, but I think that most people, when they think about the history of the Steelers, the first thing that comes to mind is Mean Joe Greene.
I don't think it makes sense to argue based on which players had how many rings, or even which players held or still hold positional statistical records, because you can pretty much expect all records to eventually be broken, and then the new records will be broken, and so forth. So to me, unless the statistics represented a singular, landmark achievement for that era of the NFL (like Marino's passing records), which in and of themselves identify the player as an "all-time" face of the team, statistics don't have any bearing on retiring a jersey.
Meaning no disrespect to Ernie Stautner, he was out of the game well before I was born, and well before the team was anything other than a perennial bottom-dweller, so I don't have any idea what he did that was so amazing that they retired his number a year after he retired from football. Again, not meaning any disrespect, but I think the fact that the earliest Steelers history in pop culture outside of Pittsburgh tends to start with the SB teams in the 70s, and you don't often find any of the retrospective shows on NFLN or ESPN talking about Stautner. Maybe that's a shame. But it illustrates the point that, even if Stautner was the greatest player in NFL history, no one really talks about him, but you ask anyone born around 1970 to name one Pittsburgh Steeler, and probably half of them will say "Mean Joe Greene" and nobody would say "Ernie Stautner."
THAT is why retiring Greene's jersey makes sense, and why I don't think you can make the same case for the other guys that also played on those 4 SB-winning teams. For guys that are still playing today or only recently ended their careers (Polamalu, Bettis) it's ridiculous to talk about retiring their numbers right now.