User blog comment:SirAston/Reviving the Dead again - A quick glance at Sion/@comment-6664150-20140114015504/@comment-5331854-20140114101319

Yes, his inability to duel pre-6 and him being very weak to getting kited makes him unable to go top against most compositions.

Seeing that Sion was one of the first champions Riot had delivered, he may be a relict from a time where jungling wasn't thought to be an essential part of LoL. Maybe Sion was thought as some kind of "battle support", a partner for bruisers. His kit looks like built for an aggressive support where he softens up someone. The single ranged stun (which is very unusual on a fighter) and his exploding shield would speak for that.