How does Riot choose the championship skin for the season?

I always thought Riot pick champions that have had a major impact in the season for instance Thresh and Janna the previous few seasons. How exactly does Riot pick which champion should they make the championship skin for in a season? The reason i ask is cause I thought this might be championship MOrgana {{champion:25}} , cause she has been a revelation this season as a strong support and found her back to competitive play and then...Riot decides championship Shyvana {{champion:102}} , who has not had a lot of impact this season. Any thought on this?
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