Champion skins unavalible for purchase sales

With the Legacy Vault being thrown open during sugar rush, I was looking forward to finally getting all the Kayle skins. But the Legacy Vault doesn't have either the Silver or Judgment Kayle skins. And I have also noticed that lots of other champs have skins that are unavailable to buy but are not in the Legacy Vault. Are we ever going to be able to buy those skins again? And if we cant buy them regularly why aren't we able to buy them during a special event like sugar rush?