The Calamity Of Demon Gods.

CARNAGE is capable of fighting hundreds of thousands of Origin gods and billions of angels and their gods at the same without losing immediately. It is even gradually shifting the tides of victory in its favor. That is not something a demon king can do. 

A demon king needs an army and maybe some allies to invade a plane. The invasion will also last numerous years. Victory and defeat won't be determined until after numerous clashes. One side will give up eventually. It could either be the gods or the demon king if they fail to maintain their strength.

A demon God on the other hand can take on a plane on its own. It is almost always a losing fight for the plane if a demon God comes prepared to take on a plane. A demon God becomes highly terrifying when they become world-enders.