Well this is sad. Illaoi was released first, but Jhin got played first

Because Illaoi is bad, both in strength and mechanic? She's only good in skirmishes, that's it. In team fights, she has no real tankiness so she gets murdered way too easily in this ADC, damage-focused meta. Plus, she lacks mobility AND hard CC, so she provides almost nothing in a competitive play.

Mechanic-wise, she's boring to play. She heavily (REALLY) relies on her opponents being either not knowing how she works or not able to dodge skillshots. Not only do pros have those knowledge and abilities (because they're willing to spend time to learn specific champion), but it's almost 100% reliant on the opponent with your own mechanic being not relevant at all (I mean, tentacle spawns, you try to land your own tentacle, and just hope your enemy can't dodge the rest of your tentacles, and if they do, you're screwed). I think a lot of people may disagree with me, but Illaoi is the worst designed champion Riot has done simply because playing her means you rely on the opponents being baddies.

I've been waiting to see someone bring out the tentacle shaman for quite a while. Every game now, it's at least 2-3 of the following minimum: Fiora, Poppy, Viktor, Corki, Lucian, Kalista, Alistar, Elise, Reksai, Quinn, Graves, and now Malphite has been added. I want to see if the professional players can use her, and if she can be a type of champion that has a flex pick option(top/jungle/mid/support).

It was the ADC patch. Teams are too busy running 3 marksmen to give Illaoi a chance to shine. I hate that champion so much but so long as there isn't a bunch of CC there isn't a way to stop her. If a team ran like Ori mid, with a zac jg or something I am pretty sure she would have enough time to get in and get the six tentacle ult and destroy. It would be great to see her in pro play though because I would like to see how she gets beaten in organized play

Illaoi sucks while Jhin is actually pretty strong. There's your answer. If Hai played a protect-the-carry support like Adrian did, then things would have turned out a lot different. You may not have noticed but Jihn was out trading Kalista except Kal had Braum's shield to soak damage and eventually just got fed enough to 1v5 people. Jhin will see more play as time goes on I assure you.