Muramana/Seraph's on Ryze

I was just browsing through probuilds.net to look at what people are building on Ryze recently and not one of them built a Muramana on him. I still love this item and it seems to add some good damage but I'm just wondering the reasoning by the pro's which makes them all go for Seraph's over the Muramana? Has it got to do with the physical damage change? Since magic pen wont effect its damage but surely in some cases mixed damage would be harder to itemize against.