First off, that is what riot is calling this new skin line: "Immortal Journey". Just figured I'd clear that up.
I'm going to copy and paste an interesting comment from Reddit from the user Warscythes;
_"I would like to offer a bit of perspective on the skins here._
_I am actually not surprised at all that Shen did not get the skin. From what I can understand, the audience for this skin line is for the Chinese players, since it is based on Xianxia. The characters in Xianxia, or at least most of them are agile characters with blades or almost sort of mystic character use chi to guide swords etc._
_Which is why Shen is actually not a good fit. It would be akin for them to make a heavy armored cowboy skin._
_Many players on the Chinese forums have guessed Yi and Fiora to be chosen, while some debated Taliyah since you can change her stone throwing to swords instead. Irelia could also be a good fit but I think there is a rework for her coming. Janna while is not exactly a huge contender but the theme of which she represents is very prevalent in Xianxia so is not surprising at all.
_Basically is a cultural thing."_
If you look at it this way, it actually makes perfect sense that these came out. Believe me, I'm just as surprised as any of you at these skins, especially with Yi. I was under the assumption that Riot stays away from a champ for at least 6 months before they start work on a new skin.....but this throws that right out the window.
However, do remember that Riot has been stepping up in production lately. This could actually mean that they are starting to pump out even more skins at a faster pace. I could be wrong, but if I'm right, it means 2018 is going to be PACKED with high quality skins.
Just a guess though.