We know king maker have bigger picture then Bal... Lets start kings road from human world arc... After hard time under Bal, Mephisto will show them king better then Delkira, because now Iruma would be comparated to Delkira and we know Iruma will be sentimental menory of old good time under Delkira... So we need Bal as ruler to let them try h*l and start them miss for old demon world;)