Old (technically current) Sion has some very serious issues with kiting. He can be fed to the point of being able to 1v5 a CCless team, but against two people with CC he can do literally nothing. His entire kit is designed around sticking to one or more targets and hitting them.
Don't get me wrong the current Sion can still do some work, but it is rather easy to outplay him since he has no play making ability. Riot could either keep a mostly useless champion around or use it as an idea for something new. If this new kit was a different champion, with all of current Sion's strengths, but none of his weaknesses Sion would see even less play.