Tristana was released in 2009, basically with the game's launch. Her latest skin brought her up to 9 skins total, with 3 of them being legacy. Her only skin prior to her rework in 2014 that had any higher quality was a 975 skin - everything else she had was 520 and 1 severely outdated "legendary" skin. Even after her rework, they only changed the particles on Firefighter (a supposedly Legendary) and called it a day. Her last 3 skins have been extremely high quality, and that's something she's needed.
Ahri, meanwhile was released in 2011, a solid 2 years after Tristana, and her skins now come up to 8, with 2 of them being legacy. Think about this. 8 skins in her 6 year long life as compared to 9 skins in Tristana's 8 year long life. Ahri has 3 high quality skins already - Pop Star, Arcade, and Challenger, and now will have a fourth. Ahri has more consistently been chosen to get a skin than Tristana, and being a younger champion with almost as many skins should tell you that. Furthermore, Ahri has 2 skins - Pop Star and Arcade - that have overlapping thematics with Star Guardian, which doesn't seem to fit Ahri as a character, other than the ties to Asia and waifus.
Soraka, meanwhile, is also a 2009 champ, and has only 7 skins, including this one coming up. There have been huge gaps between her skins, and this skin is a completely new theme for her that actually fits her thematically.