What I mean by that is that like Illaoi , Aurelian Sol is quite easy to outplay/kite/counter. I mean even when I have played Mordekaiser against Sol (on multiple occasions) I've danced arround his abilities and let me tell you - if a Mordekaiser can kite your abilities than there are some sirious problems right there. The best thing (maybe the only good thing) is that hes really really good at ganking other lanes , but a champ that is good mostly in 2v1 or 3v2 (in his favour) situations really isnt good. I am not saying that hes bad , I am saying hes easily countered/kited just like Illaoi.
Maybe I am lucky to have never met a good Illaoi/Aurelian , but you have to admit that very little people play them because of how easy it is to avoid them.