After all, a physical constitution check isn't like a willpower check—aside from directly ingesting poison with one's mouth, attacks that pass the physical constitution check had already struck and injured the holder.
And will-based checks are often far-reaching spells that can completely turn the situation around with just one failure.
Be it an outright instant-death Spell that kills if failed... or spells that manipulate minds to make enemies swap sides, forcibly dissolve hostility, or plunge someone into extreme panic, they all immediately affect the situation.
But, once reaching the Gold rank, the ability to use Elemental Power becomes extremely useful.