I think Oda did it on purpose, since he want his hybrid kinda different to his base form.
You can see Oda loving to draw Base King, I assume he choose this way since for awakening later he can show a mix hybrid version of both base and zoan to make King even looking moore beastly, like this:
A version where he have both wings and both hands to use them to protect himself and speed up.
I think we going to see King true form when he going to use awakening.