User blog comment:Emptylord/Season 3 - PBE Patch Notes/@comment-89.106.167.109-20121123102646/@comment-3974211-20121123123955

Because Ryze wants the +8% mana scaling on his Q and W and auto-attacks obtained from the Manamune upgraded.

Ryze will buy Manamune first (between the two, if you're getting either); anyone who doesn't is an idiot. It also wouldn't be a bad thing to buy the other on him - more mana, more AP, and 25% of 4000 mana as a health shield.