lulu was a good top laner because she brought a lot of teamfight utility. ulting carries or whoever is engaging, shielding low hp targets, movespeed buffs and targeted CC.
if you look at other toplaners you will notice similar characteristics and heavy focus on being either a frontline champion or bringing a ton of teamfight utility
EX: Rumble, Azir, Irelia, Gnar, Lissandra
Ahri has none of these. All she can offer her team in a fight is a moderately high cooldown CC (on a straight line, dodgeable skillshot) and some moderate to low damage.
As a split pusher she loses to other split pushers because she's not a great duelist and she doesnt deal much damage to turrets. So you don't have that benefit either.