I don't think it's just fatality rate, but also how long it takes to kill you. HIV is a great example of a disease that (untreated) has near 100% mortality rate, but can spread because it takes years to kill you.

The real issue with HIV is that you can easily spread it before being symptomatic, so far we haven’t seen hantavirus spreading before folks become symptomatic. The strain that spreads through humans has been active in south America for a while as well and hasn’t really gone anywhere yet.