The ranking system is champion specific and playstyle specific, if you are a support you are able to stay relevent through kills and assists (you would want the carry to have the kills.) and you giving CS over to the carry leaves you with close to no gold compared to the rest of the team. The ranking system is ranked by:
Gold
Creepscore
Kills
Assists
Damage dealth/taken
(participation ^)
warding? Not sure.
Even if I have an 85%-90% kill participation on playing Zilean, I don't get even an S because I have 0 CS, 0 damage because I built only CDR, mana and maxed my E-W lastly is my Q. Unlike Annie support, Brand support, etc, etc. So the issue really isn't the players, it is the ranking system. At least this is the case for me, because I play Zilean support much differently than other people do, and it is currently my main. And has been my main for quite some time . . . still only one blue out of three to get rank 7. -Sigh-