The reason Ornn's winrate is so low is because he has so few "upgrades" for his teammates.

Lots of champions have core items and builds they cannot deviate from. So, as an example, lets say you are a tank.

You want defense. You are playing a tank build, not a bruiser build. You only have 2 options really: Forgefire Cape or Infernal Mask. But if Sunfire Cape is not one of your regular armor items, then building Forgefire Cape will ruin your build. And if Abyssal Mask is not one of your regular magic resist items, then building Infernal Mask will ruin your build. Why? Consider this scenario: you normally only build one magic resist/health item. That item happens to be Spirit Visage. So in order to take advantage of Ornn's passive, you will have to build Infernal Mask instead of Spirit Visage - and despite its higher stats, it might end up being less effective and get you killed because now you're giving up the healing passive (ie: you have less effective health).

And it's not just tanks. Playing an on-hit Rageblade abusing monster? There's really no item for you. Sell your defensive item and buy Trinity Fusion I guess? And even then, since you're an on-hit champ and not a Triforce champ, this might end up getting you killed and losing the game.

I actually don't like having an Ornn on my team because I don't play champs that benefit from his items. Guinsoo's Rageblade, Blade of the Ruined King, all of the jungle items, and many more - all of them should have Ornn upgrades, so that Ornn's passive can actually benefit his entire team. And then he can be buffed/nerfed accordingly, and his winrate can finally hit 50%, because his winrate won't be tied to his teammate's ability to make use of his passive (team comp).