someone as cool and powerful as Sion
well someone's gonna have to break the news to you i guess. He's not.
The one bad thing about riot's lore is that they gave in to the whole (demi) god/space dragon bs that plagues so much content (powercreep lorewise you might say even). Sion is just not very powerfull compared to the world he resides in. (i still think the lore is pretty good in general, just not a fan of "we're a bunch of all powerfull beings and zoe did it all for us")
Sion himself. Is he cool? I think he's a pretty cool character who has a more tragic side that's not really explored. But league's kinda oversaturated when it comes to the bigass warrior dude trope. The only thing that really sets him apart from the other buff bro's is that he's undead. and i feel like most people looking for an undead character either want something like karthus, pyke or thresh. And sure since pyke release he's still one of the only 2 who are "zombies" (as opposed to spirits and a lich) but he's more warrior dude than zombie.
I honestly don't know if ryze is popular or not but he's one of the original characters and lorewise one of the most important characters, who isn't allied with anyone or anywhere and thus can meet alot of other characters (ezreal for example can do this too imo) which leads to great cinematic material.
But the most important thing is that his lore was outdated and he needed the new spot (think back to varus) in the world. Sion doesn't need new lore and got quite a bit of promo material on release, riot is alot more advanced at this point.
tldr:
He's not that popular or powerfull (compared to the world's strongest) and falls into quite a saturated group of characters. ryze Needed a new lore, like varus did and making stuff like this has most likely never been as easy for riot.