Why has Sion fallen out of favor?

He was already so much popular, even had to be nerfed. He was popular top and jungle, and was one of the few tanks used on an era were tanks were unpopular and considered to be very weak. He was one of the few tanky top laners who could whitstand ranged champions, and even though damage wise he falls of late game, he is considered to scale extremely well into an hyper tank that is incredibly difficult to kill.

So, why? Why he isn't as popular as before? Even Maokai is still somewhat popular top.