Championship riven?

Because Championship Riven is this year's reward for perfectly guessing the results of the LCS Worlds.

Its because championship riven was initially sold with the premise that she'll never come back and since championship riven was only in the store for a very short time,the skin has become incredibly valuable,so they hesitate to bring her back,which is why she is the "perfect guessing-reward" However this doesn't apply to the other championship skins.