With force or with election, who get the post of governer can't hold it without Mikado's authorization. Mikado is only family rule the Japan. Metahumans Metahumans especially orks and trolls were enemy of human in ancient days. Then it's disagreeable to live with them. It's bad to allow them behave along their nature.
Modern education make most of their harmfull nature beyond controll.
Elves and Dwarves are not so harmful as Orks and Trolls, but not all their interests are coincident with humans. Yamato-Minzoku as pure race include no metahumans. Metahumans in Japan are foreign visiters.