I mean you guys made a skin for Jhin, who came out after Illaoi. And the champs that came out before Illaoi have skins already.
I can't forget about 
this guy too. Meanwhile, Lee Sin is getting a legendary (or ultimate, we don't know yet) when he already has 7 skins.
There are several factors that limit certain champs from getting skins (based on hearsay around the boards and general logic - nothing "official")
Some champs have additional elements that need to be "skinned" as well - Zyra plants, Illaoi Tentacles, Yorik ghosts ghouls, etc. Since these are generally things that the champion and other players interact with, they must be visually distinct from the map, identifiable as the thing they're supposed to be, and look different from the base/other skins. In some cases, using the base model is possible (some of Zyra's skins I think do this) but that takes away from the skin's feel.
Some champs also have old models/rigging that really limits what a skin can do on them. Old twitch waited a really long time for skins because his old model was just terrible. Rumble has a similar problem - he only got a legendary because they made a whole new model for it. Several other champs can fall into this category, especially older ones.
Finally, some champs just don't lend themselves to skins. Zac is a good example - how do you make a green goo monster maintain the same shape/appearance but also not be a recolor? His pool party skin is well done, adds some splash effects and new stuff to his model while looking like him.
Additionally, for all champions and skins, there needs to be some spark of an idea that catches the team. Some of these champs are difficult to come up with ideas for (that actually make some sort of sense and look good in game). Crazy Cat Lady Yorik is a really fun idea, but will the Ghouls and Maiden really look that great as cats? Pool Party Illaoi sounds great, but will her tentacles look right as pool noodles? And I don't mean like, look like a cat, but fit the visual outline of the unit. For example, Zyra's plants in her different skins all have the same shape as the base model. Can you turn a Ghoul into a cat and have it do the things it does and look right? Illaoi's tentacles are kinda conical - what pool noodle is? Now, I don't have answers (and I feel like these issues can be worked around/adjusted to look OK) but there are many things about making skins that can become very difficult to actually implement.
It's most likely a legendary, given that elementalist lux came out relatively recently.
Vik, cass, swain, yorick, illaoi, rumble, and xerath are due for some new skins
Dunkmaster Illaoi needs to happen.
We need Dunkmaster Illaoi! Her idol would be like a ball Her tentacles could look like hoops When she uses ult it would make like a buzzer sound or something (idk what sound they use) And when she pulled a spirit it would show her stealing a ball Would probably be a legendary skin
Boop 
Boop
Boop...
...
...
BOOP!
I hope we get something like Flamebrand Illaoi, where her tentacles are huge pillars of fire and scorch anything in their path. Her ult would feel like a damn meteor on the rift.
They probably can't figure out what to design for her, artist block is a thing. You would think freelance artists could at least spark something. Granted, if they pay attention to any of them anyway.
Riot's way or the highway i guess.
She aint got massive boobies, a 4-inch waist and she doesn't look generic. She's generally just not marketable. Trust me, it's gonna be A LONG Time before she gets a skin.
The Skins team is pretty bias in their skin choices, and pretty poor at tying in those choices to on-going things. It'd be nice for the Demacia event to tie in with Demacian champions receiving skins, but it's never been the case.
The skins team is good when it comes to executing things (Albeit they do tend to cling to classic assets too much), but are historically pretty terrible at decision making.
Illaoi's popularity is actually less than Galio's right now, who apparently has a rate so low he needs a rework (He's actually banned like three times as much LMAO).
http://www.lolking.net/champions/illaoi#/statistics http://www.lolking.net/champions/galio#/statistics
So all things being equal and there being justice in the universe, Illaoi will get a rework before she gets a new skin.
Also this is an ancient problem and has been complained about for years. You're probably going to be upset about this for a very, very long time and even if Riot does finally make a new Illaoi one... you'll be complaining about it with another champion soon enough.
Well.. Jhin>Illaoi, of course.
Not an Ultimate, a Legendary. ONe ultimate a Year at the end of the Year.
"No"
That's Mexican, or as I personally like to call it, Spanish, for the English word "No"
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https://twitter.com/LeagueOfLegends/status/844235159104708608
another skin for Darius! :D
Even if she was, they wouldn't be able to tell you :T, and making posts about it wouldn't really rush/slow down production of one.
I thought they would do a dunkmaster illaoi with March madness. Guess that's not the case. In fact Darius is getting yet another skin before her. Yorick needs a skin too, but I'd rather they not rush it out and give him some crappy skin to prevent him from getting a good one anytime soon.
Wondering about Sol too its cool being able to say i have all his skins but when its only one it makes me cry for the sassy dragon
