That is why. You never see him. So people don't know how to anticipate his moves or play against him. But he's pretty strong and easy to master.
He's the Tank version of Brand and the like. They are supports in lane phase, but then come midgame they are just that champion. Sion does Sion things. He can actually waveclear, splitpush, gank etc. If you know how to play him he is very versatile. But never overbearing (outside of current mage rune early game).
I for one had an 80% winrate with Support Sion last season (I stop at gold 5 0LP). He's a good way to climb to your actual elo.
It's not suddenly btw. He was at 55% the whole of last year, and before that probably also. Not played much, but those that use him know him well and wreck face.