the problem isnt ahri being gross, (she is wildly useful and variable to a team), its that what other mages arent good enough or as good as she is.
lets take 
as an example.
ori has very nice last hitting, early trading (shield), and great team fight potential with her ult.
thats all very nice, but she doesn't kill someone by herself. she needs jungler assistance to get the job done.
so minimum she has to work with someone else or with her team to make an effect on the game.
but when you look at ahri, she can kill someone in lane by herself, not from her damages, but from her poke, sustain, and gap closing potential.
in addition to her charm + ult utility opportunities of escape or offense.
and oris shield is now worthless when she just gets deleted or locked down and never gets to use it, and her wave clear is terrible because shes going to take tons of poke from other longer range champions. and because her damage just isn't enough to compensate theres literally no reason to pick ori for anything other than late game team fights.
what mages need is the potential to 1v1 someone in lane. its quite difficult to do right now with 
being popular rush items these days, its just not doable for them to kill someone or get killed really.
before the mana gimp, where players could rush a rab cap or DFG first item and get kills. and before the grail nerfs for infinite poke and wave clear, mages were powerful, you basically had to have one or focus on killing the enemy mage to do something on the map.
believe me when i say ahri is just crammed pack full of utility and can get a task done, not that she herself is the outliar, its just that she has all the tools she needs to succeed in whatever it is she decides to do. (shes actually really weak damage wise)
where others do not have the necessary utility or damages (whatever is deemed appropriate for them).
