During quarantines people are not shaking hands. I am not saying quarantines necessarily select for a higher death rate (a dead host can not spread), but for a more severe disease (the main drivers of a spread are when people ultimately develop severe symptoms).

Community viruses mutate to see a lower mortality, severity, and infectiousness, since there is no selective pressure to have an r0 much higher than 1. For a community virus there is also no selective pressure to mutate to a vastly different strain, so the antibodies for other strains don't help, whereas if the virus is confined inside a city like Wuhan, the only way to reinfect is to mutate to a vastly different strain.