why is illaoi's win rate below 50% in low elo

The classic no one knows how to play her in low elo, and everyone knows how to play against her in high elo. Therefor she doesn't do good in either.

literally her counter is to walk slightly backwards, not exactly hard to beat

The mustake you are making is assuming people in low elo know how to ult more than 1 person

We may never know, just one of the many mysteries of life. Illaoi is really not that complicated to PLAY anyway, she's more of a bother to play AGAINST, which you'd think is perfect for climbing in bronze-gold.

If only. In low elo team fights even are usually all spaced out with everyone out of position, so you don't have many people at once to ult. Everyone.e is always spread out generally not ready for team fights. Or roaming jungle for no reason or poorly aplitpushing.

http://champion.gg/champion/Illaoi

this is only plat elo, but i wouldnt call it low. And even there illaoi has a bad winrate. Granted, playerbase average isnt that high.

Even if a champion is freelo, if they are difficult to play and have big differences than most champions they'll still have a win rate under 50% unless severely over tuned. Azir for instance still had a win rate under 50% when he was extremely strong since there was an almost 20% win rate difference between inexperienced Azirs and experienced Azirs.

Average win rate isn't just about the strength of the champion, part of it is determined by how experienced the average user is with the champion. This is why new champions unless either extremely easy or overturned will have low win rates, lack of experience will deflate it regardless of whether the champion is good or not. It's also why sometimes between patches champions might jump in win rate even with no direct or indirect changes, all it takes is people finding a better build or properly learning the champion.

She's only good in low elo. Her entire counter is repositioning which low elo fail to do.