Why does Ryze have such a low winrate?

Because his ult requires an organized team.

He's a rather high skill cap champion. A lot of that is the otherwise mentioned ult usage needing reasonable teamwide organisation to get the most out of, but even simply using his different combos appropriately instead of essentially button mashing is going to make for a huge spectrum of effectiveness.

The problem is that his damage requires you to be in close range of an enemy and facing them, thus there are plenty of things that can cc/murder him from out of range (cass being a rather obvious one).

Because he's an actual high skillcap champion with a high skillfloor. You'll notice his winrate spikes pretty hard, the more games you have on him. Seriously, he makes other champions who are supposed to be "high skillcap" look like a joke.

He's always had a low winrate, people just suck on him.

because trolls exist