Because he's a badly designed champion that is nearly impossible to solo kill since all 4 of his skills can be used to escape
Much like Riven who has 3, but at least Riven is fully melee unlike Aatrox who is also ranged
People just don't want to fight him, win rate doesn't really change that, old Darius was like the top "win lane lose game" champion back in the day but I would still ban him because it was all I kept seeing while trying to learn top lane, of course then I just wound up vs Teemo so it wasn't much better
Also banned Soraka for a while not because she was OP but because she was annoying