Unsolved Mysteries

Question: Why does Prophecy have a whole Purview while Mystery stays the same?

Oddly enough, because Mystery works perfectly for its intended purpose as it is. But Prophecy didn't, really, so we booted it into the realm of normal purviews with levels.

Prophecy, in the original rules, was a terrible mess. It could only be used once per story and only did one thing, which was to give you a vague premonition of possible ideas or events from the Storyteller. It was almost functionally useless, required a Storyteller to have ironclad knowledge of what was going to happen in the story arc (so also forced Storyteller railroading if PCs started to change it from his original vision), and didn't get any more useful or powerful as you gained in Legend, no matter how many levels of it you purchased. It was perfectly awful as a purview, and one of the things we changed before we even started running the first game because no PC in their right mind would want to waste Birthright and XP points on such a useless mess.

In addition, there are so many different ways in different cultures of interacting with, learning about and influencing the future that we felt a single "here's a flash of insight" boon was very unsatisfying when it came to portraying Prophecy as a power used by very different dieties in very different ways. The Prophecy purview as we currently use it has a lot more flexibility in different future-seeing powers, from knowing when something's about to happen to making prophetic pronouncements to seeing the invisible hand of Fate in the workings of the world around you; it's a much better fit for Scion's landscape as a whole instead of trying to narrow the idea down to a single unchanging dump from the Storyteller.

Mystery, on the other hand, surprisingly didn't need most of this tinkering at all. It certainly needed to be usable more than once per story (most lame thing ever), so we altered it to be usable a number of times per story equal to your dots in Mystery, which has worked very solidly for us so far. The thing is that while prophecies take different forms and have different powers around the world, the idea of Mystery really doesn't; it might be stunted differently, from reading entrails to hearing voices to getting intoxicated to just suffering sudden brief flashes of insight, but in the end it's exactly the same thing, which is drawing mysterious knowledge out of thin air. Mystery's flexibility is in that it can ask about anything that isn't covered by Prophecy, so it already encompasses all the possible powers you could reasonably assign to it. Like Arete, it was something that, while an unorthodox idea, actually worked pretty perfectly to represent the idea it was based on, so other than making sure it wasn't a lame XP sink, we left it mostly as it was.

If you're interested in exploring alternatives, there are a few versions of Mystery out there that use boons instead of the question-system, including this one from the Scion Wiki. But we love Mystery's simplicity and perfect ability to do its job, so we're sticking with it as it is for the time being.