Beacuse Boards =/= Majority of players.
The majority usually doesn't care about winrates as long as they're somewhat reasonable.
The majority cares about fun! Even in higher Elo enviroments - although the winrate and actual power is more of a factor there.
And Ahris matchup is despite all the hate recently shown on boards at any time completely fair and quite obvious. You can pick every single Champ into her and she does not affect other lanes in a way a random other mid lane could not.
Example from League of Graphs:
Silver-Gold most banned: Zed, Yasuo, Kata, Fizz and Darius - all five have lower winrates than Ahri and Zed has a whoping 46% WR - but all five share similar obnoxious uncounterable playstyles (you can just avoid them until you outscale them, but that's not fun at all!).
Going to Plat-Dia, the obnoxious crew is still around, but their banrates dropped alot - here you can find more bans directed at actually strong Champions like Lulu and Ivern who can grant major leads to their teams by existing because people in higher Elo care more about winning instead of a fun laningphase.
Ahri still has a pretty low banrate there, because her assassin'ish playstyle is nothing that would ruin a higher elo game.
So yeah - she's not considered a problem in low Elo because of her fair matchups and she's not considered a problem in high elo because of her palystyle. Hence the low Bannrate, overall.
In the end it should be obvious that the Ahri hate-train is completely overrated. Just a few loud boarders that started complaing about something that isnt a real problem - yes she has a high WR, yes a minor nerf would fix this...
**but let's be honest doesn't League have more and bigger problems than a popular Champion with a near perfect skill-curve having a decent winrate? **
Problems like over 15 Champions with less than 2% pickrate, problems like a 38% Ryze, porblems like a Lee AND a Lucian in every second game, problems like the Support role feels like the dumpster/last hope for failed mage reworks and those who got kicked out of Mid by PowerCreeping Assassins???