Illaoi had never been picked in a premier League of Legends until Thursday. Unicorn's of Love's Kiss "Vizicsacsi" Tamás picked The Kraken Priestess in their second game against Fnatic.
Vizicsacsi managed to get a solo-kill on Fnatic's Yeong-Jin "Gamsu" Noh, who was playing Maokai.
Illaoi was released in November 2015, but she hadn't been picked in one of the major leagues since. In solo-queue, she's highly acclaimed and is even ranked in the top five top-laners on champion.gg. However, pro teams prefer more utility-based and tank top-laners, like Maokai or Poppy.
While she does do some pretty solid damage in 1-on-1 fights, her tentacles can be easily avoided and ignored in team-fights, making her not the best choice.
Black Cleaver's buff did beef her up a bit, but in the end the same problems with her persist. Even if she gets off a clean ult, the damage doesn't come through if she dies too quickly. While one could build her tank to avoid premature deaths, all her burst damage would disappear and you'd want that since she lacks utility.