Legacy Skins unavailable?

So, I was attempting to get every skin in the game, minus riot trist. (I currently have 814 Skins) And then came to a wall. Why can't I get the following: Rusty Blitzcrank, Black Alistar, UFO Corki, Young Ryze, Grey Warwick, Judgment Kayle, Medieval Twitch, Silver Kayle, Unchained Alistar, Urf the Manatee (URFwick with golden olive branch crown), and URFwick. (I did not mention the Victorious Skins, PAX skins, or Triuphant Ryze) (I have used 40 Premium Chests, 20 Masterwork chests, and re-rolled skins about 30 times (using permanent skins, as well as skin shards.))
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