Why does Ryze have such a horrible win rate sitting at 43.3% mid lane...

Because he has a high skill floor.

My best guess is that people don't understand how to play passive early on with him, or can't use his ult effectively.

'cause folks suck at him.

he is a short range mage that has to level up with mana.... alot of mid laners beat him right now and he can be picked around if he is 1st picked.. he is safer going top lane vs melee as a counter pick

same as always

He takes practice and getting used to. You sort of have to practice him to get the proper hang of him. Also some people don't know how to use his combos or try to force the use of his ult.

He takes a lot of skills to play, man.

because his ult deals no damage and he loses to hard range or hard engage. hes great at stoping mobility, but he loses out to most generalized team comps.