Force of Will

Question: Why does Guardian have the most boons that cost a Legend AND a Willpower, while all other purview's boons (for the most part) only cost legend and no willpower?

Actually, Guardian's not one of the top purviews when it comes to Willpower costs. Animal, Chaos, Justice, Magic, Sky, Stars and Water all have more boons that require Willpower expenditure than Guardian does, while it's tied with Death, Earth and Industry and only one boon away from Fertility, Fire, Frost, Health, Moon and Psychopomp. The majority of our purviews have almost the same number of boons requiring Willpower that Guardian does; only five are appreciably lower (Darkness, Illusion, Prophecy, Sun and War). I'm not sure why you're feeling that Guardian has all the Willpower boons and other purviews don't; are you perhaps only looking at Hero-level boons, or something along those lines?

Willpower is an awesome thing in Scion; it's a separate resource pool from Legend that follows different rules and is both much more static but also much easier to regain. When you're just starting out as a Hero, Willpower is like candy; you have more of it than you do Legend, and it seems to come back pretty easily, plus you don't really have to worry about pesky things like Virtue Extremities too much. Later, when you've gone up in Legend and your Legend pool is enormous, Willpower becomes the precious, precious commodity that you hoard and hate spending, trying to predict how much you'll need for powers in this scene and how much of a Virtue risk you might be taking. Because it's so different from Legend, it's an essential balancing tool for boon costs; it gives a boon designer much more flexibility in getting just the right ratio of cost to effect to really make a power perfect. We don't aim to make everything cost Willpower - that would be poor design, since many things, especially at low levels, don't need it - but we don't shy away from using it when it makes sense or is a good way to balance a power's cost.

But don't despair; Willpower is easy to lose, but also easy to gain back! Stunt like there's no tomorrow, always follow your Nature to the bitter end, and if all else fails, make friends with someone who's really good at Epic Charisma.