Old Sion was clunky. AD Sion was unplayable almost and even when he was he had 2 abilities that were useful to him. One was a constant passive and the other required you to click a button once... For a passive. He could almost play without the keyboard. Sure I guess he had the stun, but he only needed one point in that to be useful. He could easily end with most of his point unspent and be largely the same.
AP Sion was good, but had some big design flaws. Why would AP Sion ever care about his ult? He didn't. He almost didn't even care about the E either, but took it last to gain some health. He could easily end the game with 3-8 unspent ability points and be largely the same. Not to mention one of his abilities was a point and click stun, which they are moving away from.
Old Sion passive was also completely random and totally invisible. When did you ever go "whew, that passive saved my life"? Never. You couldn't even tell when it did anything. There was no indication it was ever used. At very least Fizz's is constant so you know it's doing something.