Diavolo is the ruler of all demons, and all who reside in the Devildom know of his name. Although his father, The Demon King, is still alive, he has no interest in the affairs of the three worlds and has since then allowed Diavolo full reign over the Devildom.

As a result, it is taken as truth that Diavolo will be crowned as the Future King of the Devildom. As such, Diavolo has begun strengthening the Devildom's relationship with both the Human World and the Celestial Realm through the exchange program, as he believes that the three worlds should live harmoniously with each other.

(Source: Obey Me Wiki)