Basic evolution makes viruses less deadly over time. A virus that kills its host kills itself and any chance to replicate.

An evolutionarily ideal virus has 0 symptoms and a long shedding period. That means we never isolate from the sick for maximum transmission of copies it made of itself. Remember, more copies of my genes is all evolution is optimizing for.

SARS was a great example. It never fixated in society because it had no period where the sick shed the virus without symptoms so it had the bad habit of killing off the host before it spread to its next victim. Covid was closely related but it had a long shedding period before symptoms so you could spread it before you knew you were sick and it was less likely to kill you so you have more time to spread it.