I also play a lot of Sion. I think his kit is fundamentally very strong, but the meta isn't great for him right now. A lot of the popular carries are either too long range (Lux, Caitlyn) or too mobile (zed/lucian/leblanc/ezreal) to hit with his crowd control.
There are also a lot of popular bruisers right now. And a lot of damage champions are building sort of heavy. Sion has never matched up well against bruisers. He does better against teams that are either all squishy, or running one or two dedicated tanks with a squishy backline.
Finally, there are a few individual champs who are relatively popular right now who give him a very hard time. Vayne, especially. Also Kog.
Tanks are definitely strong at the moment. But most of the tanks that are seeing consistent play share one thing in common: reliability. Naut and Malphite both have much more reliable CC/initiation. Which is more valuable against a very slippery backline.
I don't know, that's what I've noticed at least. But maybe I'm not playing him right or something. I never would have expected Naut to become such a top pick. maybe Sion is in store for the same thing.