Why Should't Link Be Able to Use Feirce Diety Anywhere?

[Nye Pendragon]

Because it would be a pain to program it (he's flipping HUGE!) and the Fierce Diety mask was much more of a 'bonus' mask than anything. You didn't /have/ to obtain it at all; the other transforming masks were essentual to the plot.

So I guess Nintendo just wanted to keep it simple? IDK. :x