Heavy Is The Crown.

Kings of demons are also immortal as long as their plane still exists. A plane is not as durable as the abyss and if a demon king can create a plane, then another demon king can destroy it. That means that kings of demons will resurrect as long as they are strong enough to defend their plane. It is going to be tough because if they can't defend themselves against an assailant, odds are that they can't defend their plane from that assailant either.

The situation is not exactly hopeless. A demon king will be stronger in their plane and it requires more power to destroy the plane than to kill the king but the problem of death remains very real. The abyss is an intangible target while a plane is a tangible target that one can take their frustration out upon.