Illaoi and aatrox have the same problem.
They bring absolutely nothing to a team that their peers don't. They're both trying to be jack of all trades, but it doesn't work. A champion that brings nothing but damage to a team needs to reliably bring that damage to a fight regardless, and neither of them achieve that. Aatrox is outclassed completely by similar light divers. Master Yi is a split push beast, tryndamere is also a split pushing monster, plus his W brings great utility, xin is basically aatrox but better, fiora has awesome teamfights. Aatrox just dives and dies. He builds tanky and does fuckall or he builds damage and dies before he can do anything.
Illaoi has the same exact problem, sure she has the damage, but she brings nothing interesting to a team that other more reliable juggernauts don't. Garen becomes one of the tankiest champions in the game, with free built in armor shred, a silence, and a very useful execute. Darius is a nightmare of a juggernaut that is pretty much always useful unless he's miserably behind. Yorick is one of the most terrifying split pushers in league. Illaoi has damage, and that's basically it. And even though she has damage, she doesn't scale well at all. There is no reason to pick her over any other juggernaut.