My guess is that players that were used to playing Ryze are struggling because he lost a lot of his survivability. His shield is weaker and more situational and his spellvamp is gone. He also needs to engage before he gets his speed boost, meaning he no longer has a good way to engage.
I also am guessing it will take a while for players to get used to his new ultimate, which now occupies a lot of his power budget.
It'll be interesting to see what happens if they don't touch him for a patch or two. If they do, I hope they look at aspects other than his damage. If his win rate stays low, I think giving him more utility is a better solution than more damage.