Why I Quit Being the Demon King

The Era of Chaos.
For 665 centuries, the Demon King had invaded the human realm.
It was the brave Hero who put an end to that relentless war.
Through their sacred sacrifices, humanity barely survived.

However, this is the tale of a new Demon King.

Demon King Demiourgos, the 666th.
"I quit being the Demon King."
With a single proclamation, he abruptly left the demon realm.
This is a story of a runaway Demon King and a Hero who becomes his subordinate.
Why did he quit being the Demon King?