The characters with the most hax usually *can't* be the main characters for precisely that reason. If they're not kept in their figurative place, they can quickly become Sueish. Pluto & Saturn have ridiculous power but can't use it wantonly (their most powerful abilities have severe consequences on themselves). IF they COULD, you'd have to come up with villains who could equal them or risk throwing the series so far out of balance that it wouldn't be entertaining.
Of course, Usagi herself is no slouch in the hax department.