Viruses “attenuate”. Remember Pastuer and Rabies? It’s a natural thing viruses do. They become less virulent with each passage.
The ones that kill their host the fastest often die out with their host.
The ones that don’t kill or cause complications that kill can last and spread the longest.
The whole idea of life is to propagate. No one organism can live forever; all live through their progeny. Hard to propagate when you kill your entire world (the infected person).
A pandemic breeds itself out of the deadliest strains.