If one asked the Supreme Sea Kingdom and the Supreme God Empire for the main reason as to why they sent their princes to a mere world fragment, one would be surprised to know that even the two super powers have no concrete reason as to why they did this.
Unlike Corey and The Clown who knew about the presence of the Book of Death, none knew about the Book of Death... Even the Supreme Sea Kingdom and the Supreme God Empire.
The reason why these two organizations chose to send one of their several princes was because of a Supreme Order they received after the appearance of the fragment of Orelith.
The Supreme Order was an order majority of the outside organizations had no knowledge of. Even some individuals in the Supreme Sea Kingdom and the Supreme Celestial Empire had no knowledge of it.
It was an order that must be followed no matter what, and once it was given, even the King and Emperor had no choice but to obey the order.