Because Illaoi has not received any skin beyond her release skin.
And because this skin is chosen by the community. Riot released 3 skin concepts between 'Astronaut' Illaoi (space horror themed with alien tentacles), 'Adventurer' Illaoi (Indiana Jones themed, kinda, with vines as tentacles), and 'Battlecast' Illaoi (self explanatory, though Illaoi herself is not a machine but a resistance member who opposes the machinisation, a little bit like John Connor), for players to vote for. Whichever side you are on, 'Battlecast' Illaoi won and so it was decided that it is her new skin, now named "Resistance Illaoi" I believe. Hence Riot is constantly updating the progress of the skin's development.
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As for why 'Battlecast' won against 'Astronaut' and 'Adventurer'? Don't ask me. And don't question the integrity of the voting process either, because it was possible to multi-vote (given each account has 1 vote but each person can have multiple accounts) and obviously there were some people who abused that, but hey, let's hope each side had the same amount of cheaters.