Skin Selections?

As a buyer of the newer champions I have always liked some ideas for skins for those champions. I want to know how you select which champions get skins and when? It seems like a really bad system right now considering there are champions that have come out in the past couple months that are already getting new skins and champions that have been out for a year are still stuck with their one skin. If Riot is to make skins based off of the popularity of the champion then why don't rengar, zed, kha'zix, elise, and even thresh have new skins? These are all relatively popular champions, a couple played repeatedly by pros, and yet they all have one skin (with the exception of the rare and unavailable skins from zed and elise). I would suggest setting up a system so that a champion who came out in the past couple months (Quinn) doesn't get a second skin until someone before her has.