Top Gragas?

I'm just curious, why isn't Gragas used top more? I've been playing him a bit recently and it works so well! Start with a RoA, then a Sunfire Cape (or Randuins) and Spirit Visage as your core. After that, you're free to build whatever, his kit offers him damage (W) and utility (Q (slow), E (disrupt), R (do I really need to explain?) to a point where you can either build full tank (and still do damage with the RoA and SFC), or if you have a tanky support/jungler, you can build a bit more on the damage side (I like Rylais for the bonus utility, AP and health). But shoot, I can even go with Zhyonas or Rabadon's simply because he scales decently with AP. Here is a game i played with him I while back (When I was S5, now S3): [https://www.youtube.com/watch?v=MeKaz0Pxy3E] Warning though, I'm not the most skilled player and I'm awkward on camera :P (P.S. 18:55-19:10 is my favorite part ^_^)

tl;dr - Why don't I see more Gragas top?