A common rule of virus evolution is that viruses tend to evolve to be more contagious and less deadly. The rationale here is that a dead host is not a viable vector for contagion, and if your strain kills the host, then the strain dies along with the host body.

As a result, strains that keep people in contact with each other rather than isolated at home or in a hospital are going to be way more successful and will outcompete each other, and it goes without saying that of course the more contagious the virus is generally, the more successful it is as well.