Because the old Sion was poorly designed. AP (I don't even know how it made sense with his lore anyways) had 2 abilities and could do well without having an ult. Not other mage could really function well without an ult, why should he? AD Sion really only had 2 skills too, but both of them were just massive balls of stats. There was nothing interesting to do with them.
I never defended AP Sion but AD only needed his Q and R. He WAS auto-attack based when he went AD.
Heck, I've used my abilities as AD Sion as much as any Tryndamere would use theirs.
Also, I keep hearing people say "Ball of Stats" but that analogy only seems like the banner that people are waving to justify taking away all champion's point-and-click abilities.
Which, now that I think about it, sounds exactly like Ghostcrawler.
But to return back to the topic at hand, "Ball of Stats" is usually used as some sort of brand to mark champions who more securely guarantee a win when they get ahead.
Champions that don't have to rely on skillshots so much that it would send them back below the "Doing well" bar would be considered "Balls of Stats"
Champions such as Udyr or Master Yi. Neither of them have to worry about aiming, they just do their thing and either they are successful or they're not.
It's part of their gameplay and it's alright if it is.